Clean up lilypond-version-predicates.ily
update lilypond-version-predicates example file

Start using header fields from the actual snippets file.
This is just a demo because what we want is an include file
to create the booktitlemarkup automatically.
lilypond-version-predicates: update tags

As discussed in https://github.com/openlilylib/openlilylib/pull/40/files#r14587801 and following comments.
oll-doc: recreate Eric4 project

It seems somehow the project file had been compromised,
and I thought it the most easy approach to simply recreate it.
oll-doc: Restructure data structure

Reshape the responsibility for the different data fields.
In general make the application less knowing and let the
data objects do more work.
oll-doc: Add tags and categories dicts to Snippets

Implement functions to insert a snippet in dictionaries
with tags and categories.
Merge update lilypond-version-predicates into reorganization
Merge branch 'reorganization' into doc-script
oll-doc: Read categories and tags from snippets (TMP)

This is a temporary approach to be able to get the used
tags and categories. It's only intended for the process of
converting all the snippets to the new structure, giving
hints to the snippet authors as to what tags to use.

The parsing of tags and categories is extremely hacky,
and the output is only "for now"
.gitignore: Temporarily disable doc tracking

During developing of the docgen application it doesn't make
sense to track modifications of the generated docs.
oll-doc: Generate list of used tags in /doc