GCC documentation: porting to Sphinx

["Martin Liška" <mliska@suse.cz>]

Hello.

I've made quite some progress with the porting of the documentation and
I would like to present it to the community now:
https://splichal.eu/scripts/sphinx/

Note the documentation is automatically ([1]) generated from texinfo with a GitHub workflow ([2]).
It's built on the devel/sphinx GCC branch which I periodically with the master branch. One can
see the current source .rst files here: [3].

Changes made since the last time:
- a shared content is factored out ([4])
- conditional build is fully supported (even for shared parts)
- manual pages look reasonable well
- folders are created for files which have >= 5 TOC tree entries
- various formatting issues were resolved
- baseconf.py reads BASE-VER, DEV-PHASE, .. files

I've got couple of questions:

1) Do we have to you the following cover text?
        Copyright (c) 1988-2020 Free Software Foundation, Inc.

        Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public
        License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below).  A copy of the license is included in the gfdl(7) man page.

        (a) The FSF's Front-Cover Text is:

             A GNU Manual

        (b) The FSF's Back-Cover Text is:

             You have freedom to copy and modify this GNU Manual, like GNU
             software.  Copies published by the Free Software Foundation raise
             funds for GNU development.

2) Do we want to generate fsf-funding, gpl and gfdl manual pages?
3) Do we want to preserve the current strange copy mechanism for ./gcc/doc/tm.texi.in ?
4) Do we want a copyright header for the created .rst files?

Thoughts?
Thanks,
Martin

[1] https://github.com/davidmalcolm/texi2rst
[2] https://github.com/davidmalcolm/texi2rst/actions
[3] https://github.com/marxin/texi2rst-generated/tree/master/sphinx
[4] https://github.com/marxin/texi2rst-generated/tree/master/sphinx/share