Add an extension for defining anchor-aware external links.

Review Request #8514 — Created Nov. 2, 2016 and submitted — Latest diff uploaded

Sphinx ships with an "extlinks" extension, which allows for defining
roles that map to external links. These take a URL that work as a format
string, containing a "%s". The reference is then passed into that, and
the result is a URL embedded in the page.

This is very useful, but has a flaw in that anchors do not work with
these links. If the format string has any content after the "%s", then
that content will appear after the anchor. Support for this is being
tracked upstream by Sphinx, but as of yet there isn't any indication of
a plan to implement anchor support.

This extension adds that anchor support, building upon the logic in
Sphinx's extension and adding a shim that handles anchor-safe string
formatting. It's a drop-in replacement that can be used without changing
any configuration, meaning we can remove it down the road without any
real work if Sphinx ends up fixing this upstream.

Made use of this in some new docs. Saw that the resulting links were
correct with and without anchors.