Update, write, and unify Stylesheets documentation #765
Description
The documentation on downloading the GitHub repo, how to install the Stylesheets, pre-reqs for doing so, and how to run them is quite fragmentary (as discussed #703). There are some good resources (such as TCW32), but we should have a more unified approach that brings together existing documentation (and fills in any gaps) to address what you need to run the Stylesheets, what they are used for, how to use them, how to change them, how to test those changes, and how to propose those changes to the TEI-C.