Release handbook.musescore.org as a git repo

[stupidcomputer]

Hi there,

As the title suggests, it'd be nice to have a git repo that corresponds to the contents at https://handbook.musescore.org. The upsides, as I see them, are as follows:

1. The migration to a git repo is trivial, as the handbook is based on GitBook, which is based on git. Publishing this is just a matter of creating another git repo here and pushing.
2. Instead of relying on an internal documentation team to cover materials in MuseScore, the community can also contribute documentation as well -- which might offload some of the burden of the internal team.
3. Users can files issues against the documentation, which is good for crowdsourcing deficiencies with the documentation. This is not possible under the current model, except by manually emailing certain people or creating irrelevant issues on other repos within the organization.

There is one drawback, though -- the current maintainers of the documentation must review and ensure that proposed changes are correct. Is this a cost worth bearing?