Rewrite "Using the TEI GitHub Repository"

[ebeshero]

Attempting to fix a dead link on the Stylesheets README (see #703 and #710) leads to an unhappy discovery: Our documentation about working locally with the Stylesheets is outdated.

I recommend that we rewrite the Using the TEI GitHub Repository page that now only exists on the old TEI website. Some of the information is useful, but much of it is out of date.

• We should definitely remove the outdated info about working with the old Roma.
• Maybe we can trim this to confine the advice to local installation requirements and how to work with the Makefile?
• If we make a new version of this page, we should link it to our TCW 32: Testing and Building the TEI Guidelines, which provides guidance on the Docker environment.

[ebeshero]

Self-assigning to connect with #705