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>