Use sphinxcontrib-globalsubs to define reST substitutions instead of rst_epilog #2280

As described in #2277, putting reStructuredText substitutions stored in docs/common_links.rst and storing them in rst_epilog is causing huge slowdowns in our documentation builds. As an alternative, we should use sphinxcontrib-globalsubs to define global substitutions instead.

Since we adopted this practice following Astropy, it would be worth working together to implement sphinxcontrib-globalsubs for both packages simultaneously (see also astropy/astropy#15149).

Supersedes #2277.