content/static/doc: set proper header ids in module documentation

Previously, we used a separate <a> tag before the header. Headers had
temporary ids, and the table of contents linked to those. With this
change, the header elements themselves will have ids.

For golang/go#33637

Change-Id: Ia79048f0db7cc4e8eac3bb5641278acd354fca01
Reviewed-on: https://go-review.googlesource.com/c/website/+/244766
Reviewed-by: Michael Matloob <matloob@golang.org>