Add an extension for defining anchor-aware external links.
Review Request #8514 — Created Nov. 2, 2016 and submitted
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.