Support for all-pages-on-site epilog and prolog

[pradyunsg]

Is your feature request related to a problem? Please describe.

Sphinx allows appending/pre-pending to sources of a ReStructuredText file, using the rst_epilog and rst_prolog attributes in conf.py. This is especially useful for HTML meta tags, that are shared across the site.

Describe the solution you'd like

Replicate https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-rst_epilog, but as md_epilog.

Describe alternatives you've considered

None. I think this'll be really nice to have.

Additional context

See pradyunsg/furo#31 for what chain of events led me to file this issue.

[chrisjsewell]

this is a duplicate of #190, I will explain there how #273 goes some way to addressing this