doc: add skeleton module documentation with headings

Sections will be filled in with individual CLs before Go 1.14.

NOTE: This document is currently in Markdown for ease of writing /
reviewing. Before Go 1.14, we will either ensure that x/website
can render Markdown (flavor TBD) or check in a rendered HTML file that
can be displayed directly.

Updates #33637

Change-Id: Icd43fa2bdb7d256b28a56b93214b70343f43492e
Reviewed-on: https://go-review.googlesource.com/c/go/+/202081
Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/modules.md

@@ -0,0 +1,61 @@
+<!--{
+  "Title": "Go Modules Reference",
+  "Subtitle": "Version of Sep 4, 2019",
+  "Path": "/ref/modules"
+}-->
+<!-- TODO(jayconrod): ensure golang.org/x/website can render Markdown or convert
+this document to HTML before Go 1.14. -->
+<!-- TODO(jayconrod): ensure Markdown renderer adds anchors or add them
+manually. -->
+
+## Introduction
+
+## Glossary
+
+## Packages, modules, and versions
+
+## go.mod files
+
+### go.mod file format
+
+### Minimal version selection (MVS)
+
+### Compatibility with non-module repositories
+
+## Module-aware build commands
+
+### Enabling modules
+
+### Initializing modules
+
+### Build commands
+
+### Vendoring
+
+### `go mod download`
+
+### `go mod verify`
+
+### `go mod edit`
+
+### `go clean -modcache`
+
+### Module commands outside a module
+
+## Retrieving modules
+
+### GOPROXY protocol
+
+### Module zip requirements
+
+### Privacy
+
+### Private modules
+
+## Authenticating modules
+
+### go.sum file format
+
+### Checksum database
+
+### Privacy