matchers: add missing format_any_of docs.

* index.md (Custom Matchers): Document format_any_of.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
gh-pages: fix markdown h2 anchor shortcuts.

* index.md: The Jekyll theme uses javascript to build a navigation
sidebar from h1/h2 headers, but in the process rewrites the
anchor names with '-' separators. Adjust the markdown shortcuts to
match.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>