scripts/sphinx-pre-install: always check if version is compatible with build

Call the script every time a make docs target is selected, on
a simplified check mode.

With this change, the script will set two vars:

$min_version - obtained from `needs_sphinx` var inside
	       conf.py (currently, '1.3')

$rec_version - obtained from sphinx/requirements.txt.

With those changes, a target like "make htmldocs" will do:

1) If no sphinx-build/sphinx-build3 is found, it will run
   the script on normal mode as before, checking for all
   system dependencies and providing install hints for the
   needed programs and will abort the build;

2) If no sphinx-build/sphinx-build3 is found, but there is
   a sphinx_${VER}/bin/activate file, and if
   ${VER} >= $min_version (string comparation), it will
   run in full mode, and will recommend to activate the
   virtualenv. If there are multiple virtualenvs, it
   will string sort the versions, recommending the
   highest version and will abort the build;

3) If Sphinx is detected but has a version lower than
   $min_version, it will run in full mode - with will
   recommend creating a virtual env using sphinx/requirements.txt,
   and will abort the build.

4) If Sphinx is detected and version is lower than
   $rec_version, it will run in full mode and will
   recommend creating a virtual env using sphinx/requirements.txt.

   In this case, it **won't** abort the build.

5) If Sphinx is detected and version is equal or righer than
   $rec_version it will return just after detecting the
   version ("quick mode"), not checking if are there any
   missing dependencies.

Just like before, if one wants to install Sphinx from the
distro, it has to call the script manually and use `--no-virtualenv`
argument to get the hints for his OS:

    You should run:

	sudo dnf install -y python3-sphinx python3-sphinx_rtd_theme

While here, add a small help for the three optional arguments
for the script.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>