Document sources for `@class` in `chessboard_faust_ii.svg`

[gerritbruening]

@thvitt is there a source for the class assignments in chessboard_faust_ii.svg? Or is this original data, supplied, say, on paper by the project's textual scholars?

[thvitt]

The chessboard files have been written manually and not generated. The classes refer to the inline stylesheet in the file itself:

https://github.com/faustedition/faust-web/blob/6dc72285926f2c39acacac6f487c88a44070e85d/img/chessboard_faust_ii.svg?short_path=580b6ec#L3-L59

[gerritbruening]

So the sources are to be documented somewhere, at least in the file itself, within `.