GCC documentation: porting to Sphinx

[Martin Liška]

Hello.

I've just made a first version of the patchset that sits in GCC source tree:
https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;a=log;h=refs/users/marxin/heads/sphinx-v2

Changes since the last submission:
1) I made a brief proofreading and fixed most of the formatting and other issues
2) target hook documentation was ported to RST format and gcc/genhooks generated a tm.rst.in
    that is then used via .. include:: directives
3) sphinx-build detection is implemented in gcc/configure.ac
3) make integration is done (currently covering info and man pages) for gcc and gcc/fortran targets

As before, one can see the result of generated output here:
https://splichal.eu/scripts/sphinx/

Known limitations:
1) I found a bug in man page generation that is currently fixed:
https://github.com/sphinx-doc/sphinx/issues/1860#issuecomment-861793094
Note the fix will appear in the upcoming 4.1.0 release. Without the patch, one can see wrong
font selection in a generated manual page

2) Right now, I rely on caching capability of sphinx-build. That means when no source change is detected,
sphinx-build exits immediately. However, it's not working for all targets (info, man) and I've suggested
a patch for it:
https://github.com/sphinx-doc/sphinx/issues/9359

3) I haven't prepared patch for .texi removal (should be quite easily doable).

4) the currently existing Sphinx manuals (Ada and libgccjit) are not integrated yet.

@Joseph: Can you share your thoughts about the used Makefile integration? What do you suggest for 2)
(note that explicit listing of all .rst file would be crazy)?

Thoughts?
Thanks,
Martin