Use an HTML comment with triple dashes for the copypright header, which
means that the paragraph will be ignored when rendering both HTML and
TeX.

While at it, quote "GC", and properly link to internal/ssa/README.md.

Change-Id: Ib18529d2fc777d836e74726ff1cfe685e08b063c
Reviewed-on: https://go-review.googlesource.com/109875Reviewed-by: Matthew Dempsky <mdempsky@google.com>