[refman] Move chapters into new structure.

As a first step toward a deeper refactoring of the reference manual,
we move existing chapters into a new structure.

We use the Sphinx support for top-level chapters spanning multiple
pages to consolidate existing chapters into a smaller number of
chapters and a smaller number of parts.

Now the full top-level table of content can be seen in one glance.
Most of the new chapters are divided into several sub-chapters (on
separate pages) that correspond to the pre-existing chapters.  These
new top-level chapters gathering several chapters together have gained
a new introduction.  The main introduction has been rewritten /
simplified as well.

For now, the URL of pre-existing chapters does not change.  The intent
is to further refactor the manual by splitting some of these
sub-chapters into smaller ones, and by moving things around.

While the sub-chapters are likely to evolve very much in the future,
the top-level table of content is almost final (except that the "Using
Coq" part may gain one or two additional chapters on proof engineering
/ project management).

Thanks to Jim Fehrle for investigating how to split a chapter on
multiple pages and to both Jim and Matthieu Sozeau for the discussion
that led to this new structure.

See also the related CEP: https://github.com/coq/ceps/pull/43

Additional notes:

- A new directory structure has been created reflecting the new
  chapter structure.

- The indexes chapter has been removed from the PDF version since it
  wasn't working.

Co-authored-by: Jim Fehrle <jfehrle@sbcglobal.net>

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diff --git a/doc/sphinx/language/extensions/index.rst b/doc/sphinx/language/extensions/index.rst
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+.. _extensions:
+
+===================
+Language extensions
+===================
+
+Elaboration extends the language accepted by the Coq kernel to make it
+easier to use.  For example, this lets the user omit most type
+annotations because they can be inferred, call functions with implicit
+arguments which will be inferred as well, extend the syntax with
+notations, factorize branches when pattern-matching, etc.  In this
+chapter, we present these language extensions and we give some
+explanations on how this language is translated down to the core
+language presented in the :ref:`previous chapter <core-language>`.
+
+This chapter is divided in several sub-chapters:
+
+.. toctree::
+   :maxdepth: 1
+
+   ../gallina-extensions
+   ../../addendum/extended-pattern-matching
+   ../../user-extensions/syntax-extensions
+   ../../addendum/implicit-coercions
+   ../../addendum/type-classes
+   ../../addendum/canonical-structures
+   ../../addendum/program
+   ../../proof-engine/vernacular-commands