docs: conf.py: avoid thousands of duplicate label warning on Sphinx

The autosectionlabel extension is nice, as it allows to refer to
a section by its name without requiring any extra tag to create
a reference name.

However, on its default, it has two serious problems:

1) the namespace is global. So, two files with different
   "introduction" section would create a label with the
   same name. This is easily solvable by forcing the extension
   to prepend the file name with:

	autosectionlabel_prefix_document = True

2) It doesn't work hierarchically. So, if there are two level 1
   sessions (let's say, one labeled "open" and another one "ioctl")
   and both have a level 2 "synopsis" label, both section 2 will
   have the same identical name.

   Currently, there's no way to tell Sphinx to create an
   hierarchical reference like:

		open / synopsis
		ioctl / synopsis

  This causes around 800 warnings. So, the fix should be to
  not let autosectionlabel to produce references for anything
  that it is no...