Generate HTML documentation with texi2any

The design of this merge request follows two principles:

• The intermediate state of every commit is buildable. The produced documentation may be incomplete and / or ugly, but the result at least contains the content of LilyPond's current documentation.
• The new customizations in Documentation/lilypond.init are developed in logical steps. This makes the changes somewhat reviewable, but more importantly serves as documentation for the future of what is actually required to implement which part of the customized output.

Closes #1557 (closed)