use Sphinx substitutions to manage personnel list

[gkthiruvathukal]

On https://graduatehandbook.cs.luc.edu/general.html and elsewhere on the site, we have hard-coded information (names, e-mails, etc.) We should the substitution framework in Sphinx to make it easy to change names.

For example, we could have this substitution for gpd:

.. |gpd| replace:: Andrew N. Harrington

.. |gpd-email| replace:: [email protected]

Then in the .rst files, wherever you see a mention of Andy or his e-mail, use the substitution instead.

We also need a separate substitution for our other GPD, Channah Naimann. Will add a separate issue for that.