Add the tools section with a Gofmt sub-section, just like in
go1.10.html. Instead of copying the two last paragraphs from 1.10, which
warn users about the hidden complexity of enforcing gofmt, move that to
go/format and link to it.

While at it, remove a duplicate "Tools" header that was likely added by
accident.

Fixes #26228.

Change-Id: Ic511c44b2b86f82a41f2b78dd7e7482d694b6c62
Reviewed-on: https://go-review.googlesource.com/122295Reviewed-by: Ian Lance Taylor <iant@golang.org>