qapi: Add documentation format validation

qapi: Add documentation format validation

Add explicit validation for QAPI documentation formatting rules:

1. Lines must not exceed 70 columns in width (including '# ' prefix)
2. Sentences must be separated by two spaces

Example sections and literal :: blocks (seldom case) are excluded, we
don't require them to be <= 70, that would be too restrictive. Anyway,
they share common 80-columns recommendations (not requirements).

Add two simple tests, illustrating the change.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20251031183129.246814-1-vsementsov@yandex-team.ru>

The detection of example and literal blocks isn't quite correct, but
it works well enough, and we can improve on top.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Comments, error messages, and test file names tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>