doc: adjust installation instructions dynamically for a given download

This change allows the download page to redirect the user to
/doc/install?download=filename so the user can see installation
instructions specific to the file they are downloading.

This change also expands the "Test your Go installation" section
to instruct the user to create a workspace, hopefully leading
to less confusion down the line.

It also changes the front page download link to go directly
to the downloads page, which will in turn take them to the
installation instructions (the original destination).

This is related to this change to the tools repo:
https://golang.org/cl/13180

Change-Id: I658327bdb93ad228fb1846e389b281b15da91b1d
Reviewed-on: https://go-review.googlesource.com/13151
Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/install.html

@@ -3,10 +3,12 @@
 	"Path":  "/doc/install"
 }-->
 
+<div class="hideFromDownload">
+
 <h2 id="download">Download the Go distribution</h2>
 
 <p>
-<a href="https://golang.org/dl/" id="start" class="download" target="_blank">
+<a href="https://golang.org/dl/" id="start" class="download">
 <span class="big">Download Go</span>
 <span class="desc">Click here to visit the downloads page</span>
 </a>
@@ -60,6 +62,8 @@ installed Xcode 4.3+, you can install it from the Components tab of the
 Downloads preferences panel.
 </p>
 
+</div><!-- hideFromDownload -->
+
 
 <h2 id="install">Install the Go tools</h2>
 
@@ -68,6 +72,8 @@ If you are upgrading from an older version of Go you must
 first <a href="#uninstall">remove the existing version</a>.
 </p>
 
+<div id="tarballInstructions">
+
 <h3 id="tarball">Linux, Mac OS X, and FreeBSD tarballs</h3>
 
 <p>
@@ -77,10 +83,10 @@ and extract it into <code>/usr/local</code>, creating a Go tree in
 </p>
 
 <pre>
-tar -C /usr/local -xzf go$VERSION.$OS-$ARCH.tar.gz
+tar -C /usr/local -xzf <span class="downloadFilename">go$VERSION.$OS-$ARCH.tar.gz</span>
 </pre>
 
-<p>
+<p class="hideFromDownload">
 Choose the archive file appropriate for your installation.
 For instance, if you are installing Go version 1.2.1 for 64-bit x86 on Linux,
 the archive you want is called <code>go1.2.1.linux-amd64.tar.gz</code>.
@@ -125,6 +131,10 @@ export PATH=$PATH:$GOROOT/bin
 location.
 </p>
 
+</div><!-- tarballInstructions -->
+
+<div id="darwinPackageInstructions">
+
 <h3 id="osx">Mac OS X package installer</h3>
 
 <p>
@@ -139,15 +149,21 @@ The package should put the <code>/usr/local/go/bin</code> directory in your
 Terminal sessions for the change to take effect.
 </p>
 
+</div><!-- darwinPackageInstructions -->
+
+<div id="windowsInstructions">
+
 <h3 id="windows">Windows</h3>
 
-<p>
+<p class="hideFromDownload">
 The Go project provides two installation options for Windows users
 (besides <a href="/doc/install/source">installing from source</a>):
 a zip archive that requires you to set some environment variables and an
 MSI installer that configures your installation automatically.
 </p>
 
+<div id="windowsInstallerInstructions">
+
 <h4 id="windows_msi">MSI installer</h4>
 
 <p>
@@ -162,6 +178,10 @@ The installer should put the <code>c:\Go\bin</code> directory in your
 command prompts for the change to take effect.
 </p>
 
+</div><!-- windowsInstallerInstructions -->
+
+<div id="windowsZipInstructions">
+
 <h4 id="windows_zip">Zip archive</h4>
 
 <p>
@@ -177,6 +197,8 @@ you must set the <code>GOROOT</code> environment variable to your chosen path.
 Add the <code>bin</code> subdirectory of your Go root (for example, <code>c:\Go\bin</code>) to your <code>PATH</code> environment variable.
 </p>
 
+</div><!-- windowsZipInstructions -->
+
 <h4 id="windows_env">Setting environment variables under Windows</h4>
 
 <p>
@@ -186,15 +208,38 @@ versions of Windows provide this control panel through the "Advanced System
 Settings" option inside the "System" control panel.
 </p>
 
+</div><!-- windowsInstructions -->
+
 
 <h2 id="testing">Test your installation</h2>
 
 <p>
-Check that Go is installed correctly by building a simple program, as follows.
+Check that Go is installed correctly by setting up a workspace
+and building a simple program, as follows.
 </p>
 
 <p>
-Create a file named <code>hello.go</code> and put the following program in it:
+Create a directory to contain your <a href="code.html#Workspaces">workspace</a>,
+<code>$HOME/work</code> for example, and set the <code>GOPATH</code> environment
+variable to point to that location.
+</p>
+
+<pre>
+$ <b>export GOPATH=$HOME/work</b>
+</pre>
+
+<p>
+You should put the above command in your shell startup script
+(<code>$HOME/.profile</code> for example) or, if you use Windows,
+follow the <a href="#windows_env">instructions above</a> to set the
+<code>GOPATH</code> environment variable on your system.
+</p>
+
+<p>
+Next, make the directories <code>src/github.com/user/hello</code> inside your
+workspace (if you use GitHub, substitute your user name for <code>user</code>),
+and inside the <code>hello</code> directory create a file named <code>hello.go</code>
+with the following contents:
 </p>
 
 <pre>
@@ -208,11 +253,21 @@ func main() {
 </pre>
 
 <p>
-Then run it with the <code>go</code> tool:
+Then compile it with the <code>go</code> tool:
 </p>
 
 <pre>
-$ go run hello.go
+$ <b>go install github.com/user/hello</b>
+</pre>
+
+<p>
+The above command will put an executable command named <code>hello</code> 
+(or <code>hello.exe</code>) inside the <code>bin</code> directory of your workspace.
+Execute the command to see the greeting:
+</p>
+
+<pre>
+$ <b>$GOPATH/bin/hello</b>
 hello, world
 </pre>
 
@@ -220,17 +275,10 @@ hello, world
 If you see the "hello, world" message then your Go installation is working.
 </p>
 
-
-<h2 id="gopath">Set up your work environment</h2>
-
 <p>
-You're almost done.
-You just need to set up your environment.
-</p>
-
-<p>
-Read the <a href="/doc/code.html">How to Write Go Code</a> document,
-which provides <b>essential setup instructions</b> for using the Go tools.
+Before rushing off to write Go code please read the
+<a href="/doc/code.html">How to Write Go Code</a> document,
+which describes some essential concepts about using the Go tools.
 </p>
 
 

doc/root.html

@@ -53,7 +53,7 @@ simple, reliable, and efficient software.
 
 <div id="gopher"></div>
 
-<a href="/doc/install" id="start">
+<a href="https://golang.org/dl/" id="start">
 <span class="big">Download Go</span>
 <span class="desc">
 Binary distributions available for<br>