Merge: nitx: use doc commands

Three things in this PR:
* `nitx` now uses doc-commands (and related cleaning), bye bye the heavy `DocModel` with pre built `DocPages`
* More tests for `nitx`
* Move `nitx` to the `doc` tools suite, the group is now known as `doc::term`

Pull-Request: #2591
Reviewed-by: Jean Privat <jean@pryen.org>