doc: Add a module index

[Bruno Haible]

It is hard to learn about the existing Gnulib modules.
Even I had forgotten about the 'eealloc' and 'safe-alloc' modules
over the years. And when starting to use module 'string-buffer'
in xgettext, it took me something like 15 minutes to find this
module.

The texinfo-formatted documentation covers only about half of the
modules.

The MODULES.html file is not really giving an overview, either.

So, I plan to document most modules in the texinfo-formatted documentation.
The plan is not to copy the contents of the .h files into .texi —
we assume that the user of Gnulib is able to read .h files.
The plan is instead to create new sections and chapters that
explain
  - which modules exist,
  - when to pick one or the other.
Like we already do in the chapters "Multithreading" and
"Strings and Characters".

As a first step towards this goal, I'm adding two indices to the manual:
  - a "Module Index",
  - an "Undocumented Module Index", which is kind of a TODO list.


2024-10-27  Bruno Haible  <bruno@clisp.org>

        doc: Add a module index.
        * doc/Makefile (undocumented-modules.texi): New rule.
        (%.info, %.html, %.dvi, %.pdf): Depend on undocumented-modules.texi.
        (mostlyclean): Remove also *.m and *.tmp.
        (force): New rule.
        * doc/*.texi: Add module index entries.
        * doc/*/*.texi: Likewise.

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