GCC documentation: porting to Sphinx

[Arnaud Charlet]

> I've got something that is very close to be a patch candidate that can be
> eventually merged. Right now, the patches are available here:
> https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;a=log;h=refs/users/marxin/heads/sphinx-v3

FWIW I would prefer to review the changes posted here directly with all
the details.

In particular can you explain the motivation behind all the changes in the
gcc/ada/doc directory? That's lots of moving files around, so I'd like
to understand why and make sure these are not gratuituous changes, since
move files around is always tricky and I'd rather not have to undo it
later in case this causes troubles or have unexpected consequences.

Otherwise, glad to see the switch to sphinx finally moving in gcc!

Arno