The approach of `rewrite_github_anchor` makes some anchors valid. But
it also makes other kind of anchors invalid. This disables the handler
to make them valid again (while 4.1.x release).
To describe the purpose more accurately, the `master_doc` is now renamed
to `root_doc`. The old name is still available. But it is recommeneded
to use new one from now on.
To avoid needing to turn off anchor checking across the entire
documentation allow skipping based on matching the anchor against a
regex.
Some sites/pages use JavaScript to perform anchor assignment in a
webpage, which would require rendering the page to determine whether
the anchor exists. Allow fine grain control of whether the anchor is
checked based on pattern matching, until such stage as the retrieved
URLs can be passed through an engine for deeper checking on the HTML
doctree.
