Add sphinxcontrib-globalsubs

[namurphy]

The sphinxcontrib-globalsubs extension is awesome because using it let us speed up our Sphinx build by a factor of two!

For a bit of background, the Sphinx documentation states that you can add global substitutions via the rst_prolog or rst_epilog configuration variables (which contain text that will be prepended or appended to every reStructuredText source file). However, we didn't realize that adding ∼200 substitutions in rst_epilog would significantly slow down our documentation builds.

After finding sphinx-doc/sphinx#2173, I learned about sphinxcontrib-globalsubs. After defining the substitutions with this extension instead of rst_epilog, I found that our documentation build dropped from ∼415 s to ∼200 s!