First of all, thanks so much to all involved in making Roma!

Now, I might be just me being unable to find it, but is there any documentation out there? That would be immensely helpful!

I can't see anything linked from the start page, and the section in the Guidelines on Customization also does not cover this [1], which is of course reasonable. TEI by Example is great and has a chapter on Roma [2], but it describes version 0.28, while we're currently at 1.30.0. I'm unfortunately too much in need of the documentation to be much help in creating it, but would happily volunteer to test it...

[1] https://tei-c.org/release/doc/tei-p5-doc/en/html/USE.html#MD
[2] https://teibyexample.org/exist/tutorials/TBED08v00.htm#minimal