doc: change anchor URLs from golang.org/dl/ to /dl/

When running locally, it will redirect properly to golang.org/dl/
(see https://github.com/golang/tools/blob/master/cmd/godoc/dl.go).

This is to support domains in prod other than golang.org.

Change-Id: I6d3051fcd7e06a86442324a64d781d8ad95c624f
Reviewed-on: https://go-review.googlesource.com/88679
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.html

@@ -2035,4 +2035,4 @@ They are available for many combinations of architecture and operating system
 Installation details are described on the
 <a href="/doc/install">Getting Started</a> page, while
 the distributions themselves are listed on the
-<a href="https://golang.org/dl/">downloads page</a>.
+<a href="/dl/">downloads page</a>.

doc/install.html

@@ -8,14 +8,14 @@
 <h2 id="download">Download the Go distribution</h2>
 
 <p>
-<a href="https://golang.org/dl/" id="start" class="download">
+<a href="/dl/" id="start" class="download">
 <span class="big">Download Go</span>
 <span class="desc">Click here to visit the downloads page</span>
 </a>
 </p>
 
 <p>
-<a href="https://golang.org/dl/" target="_blank">Official binary
+<a href="/dl/" target="_blank">Official binary
 distributions</a> are available for the FreeBSD (release 10-STABLE and above),
 Linux, Mac OS X (10.8 and above), and Windows operating systems and
 the 32-bit (<code>386</code>) and 64-bit (<code>amd64</code>) x86 processor
@@ -33,7 +33,7 @@ system and architecture, try
 <h2 id="requirements">System requirements</h2>
 
 <p>
-Go <a href="https://golang.org/dl/">binary distributions</a> are available for these supported operating systems and architectures.
+Go <a href="/dl/">binary distributions</a> are available for these supported operating systems and architectures.
 Please ensure your system meets these requirements before proceeding.
 If your OS or architecture is not on the list, you may be able to
 <a href="/doc/install/source">install from source</a> or
@@ -77,7 +77,7 @@ first <a href="#uninstall">remove the existing version</a>.
 <h3 id="tarball">Linux, Mac OS X, and FreeBSD tarballs</h3>
 
 <p>
-<a href="https://golang.org/dl/">Download the archive</a>
+<a href="/dl/">Download the archive</a>
 and extract it into <code>/usr/local</code>, creating a Go tree in
 <code>/usr/local/go</code>. For example:
 </p>
@@ -138,7 +138,7 @@ location.
 <h3 id="osx">Mac OS X package installer</h3>
 
 <p>
-<a href="https://golang.org/dl/">Download the package file</a>,
+<a href="/dl/">Download the package file</a>,
 open it, and follow the prompts to install the Go tools.
 The package installs the Go distribution to <code>/usr/local/go</code>.
 </p>
@@ -167,7 +167,7 @@ MSI installer that configures your installation automatically.
 <h4 id="windows_msi">MSI installer</h4>
 
 <p>
-Open the <a href="https://golang.org/dl/">MSI file</a>
+Open the <a href="/dl/">MSI file</a>
 and follow the prompts to install the Go tools.
 By default, the installer puts the Go distribution in <code>c:\Go</code>.
 </p>
@@ -185,7 +185,7 @@ command prompts for the change to take effect.
 <h4 id="windows_zip">Zip archive</h4>
 
 <p>
-<a href="https://golang.org/dl/">Download the zip file</a> and extract it into the directory of your choice (we suggest <code>c:\Go</code>).
+<a href="/dl/">Download the zip file</a> and extract it into the directory of your choice (we suggest <code>c:\Go</code>).
 </p>
 
 <p>

doc/root.html

@@ -58,7 +58,7 @@ simple, reliable, and efficient software.
 
 <div id="gopher"></div>
 
-<a href="https://golang.org/dl/" id="start">
+<a href="/dl/" id="start">
 <span class="big">Download Go</span>
 <span class="desc">
 Binary distributions available for<br>