Purge on broken links within the (HTML) documentation. This mostly involves
deleting SEEALSO entries (where documentation doesn't exist) but also fixing a
few links.

Also fixed the anchor links in TheoryIndex.html and idIndex.html. In both
cases there aren't any entries starting with the letter "Y".

make the documentation of proofManagerLib reflect the renaming of BBackup to restore and save_proof to save

Documentation for the new proofManagerLib procedures.

I added some brief documentation for save_proof, Backup, forget_history, and
restart. These could all still benefit from some examples.

Unfortunately subversion says proofManagerLib.backup.doc and
proofManagerLib.Backup.doc clash, so I saved the Backup documentation in
proofManagerLib.BBackup.doc. If this gets fixed somehow, then the SEEALSO links
should be changed too.

I changed the SEEALSO section for all the proofManagerLib contents to refer to
everything in the signature (excluding prettyprinting stuff and abbreviations).
Exceptions: expand and backup also refer to their abbreviations, and expand(f)
also refers to Tactical.VALID as before. And I refer to BBackup instead of
Backup for the reason above.

Some of the links are broken (some were broken before this change too), since
there isn't yet documentation for gt, set_goaltree, new_goalstack,
new_goaltree, add, drop, dropn, et, expandv, initial_goal, top_goals, status,
rotate, or rotate_proofs.