Revert Sphinx documentation [Was: Issues with Sphinx]

["Martin Liška" <mliska@suse.cz>]

Hi.

The situation with the Sphinx migration went out of control. The TODO list
overwhelmed me and there are road-blocks that can't be easily fixed with what
Sphinx currently supports. That would require addition of an upstream support and
a possible new Sphinx release.

Let me summarize the biggest road blocks:

1) PR107634 - documentation is divided among much files than it used to be; plus
   the current filenames tend to be very long
2) Index page regressions: PR107643 and PR107651
3) PR107656 - c::macro and c::function does replace [Macro:], [Target Hook:] well
4) Makefile.am, build system issues: missing support for lib*/Makefile.am and various
   limitation when it comes to 'make install-*'. Moreover, gcc_release --enable-generated-files-in-srcdir
   is not supported yet.

Plus, there are other issues linked in PR107655 and we face the issue that
many web links (to GCC documentation) are in the wild and should not become 404.

I would like to apologize to anybody who wasted a time with adoption to the Sphinx format
which we be reverted eventually. Special thanks belongs to all people who helped me and
prepared various patches.

I'm going to revert the patchset during today (Monday) and I'll send a patch with a couple
of new changes that landed in the period of time we used Sphinx.

Thank you for your understanding.
Martin