docs: remove unneeded maintainer_handbooks_main label

docs: remove unneeded maintainer_handbooks_main label

Somehow people got into the habit of putting labels at the tops of
documentation files, even when they are not used.  It is better to just
give the name of a file when linking to the whole thing; remove the label
and update the references accordingly.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>