cmd/go/internal/doc: clean up after merge with cmd/internal/doc
This is done in a separate CL to reduce the diffs from the previous CL.
Merge the main.go and doc.go files, and isolate the bootstrap-tagged
code to one file.
For #74667
Change-Id: I11bf0aa18beeb898937135f49f473c1ba1b7e756
Reviewed-on: https://go-review.googlesource.com/c/go/+/689875 Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>