[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>

2 files changed, 44 insertions, 0 deletions

diff --git a/doc/sphinx/appendix/history-and-changes/index.rst b/doc/sphinx/appendix/history-and-changes/index.rst
new file mode 100644
index 0000000000..3535a4d8a7
--- /dev/null
+++ b/doc/sphinx/appendix/history-and-changes/index.rst
@@ -0,0 +1,20 @@
+.. _history-and-changes:
+
+==========================
+History and recent changes
+==========================
+
+This chapter is divided in two parts.  The first one is about the
+early history of Coq and is presented in chronological order.  The
+second one provides release notes about recent versions of Coq and is
+presented in reverse chronological order.  When updating your copy of
+Coq to a new version (especially a new major version), it is strongly
+recommended that you read the corresponding release notes.  They may
+contain advice that will help you understand the differences with the
+previous version and upgrade your projects.
+
+.. toctree::
+   :maxdepth: 1
+
+   ../../history
+   ../../changes
diff --git a/doc/sphinx/appendix/indexes/index.rst b/doc/sphinx/appendix/indexes/index.rst
new file mode 100644
index 0000000000..a5032ff822
--- /dev/null
+++ b/doc/sphinx/appendix/indexes/index.rst
@@ -0,0 +1,24 @@
+:orphan:
+
+.. _indexes:
+
+========
+Indexes
+========
+
+We provide various specialized indexes that are helpful to quickly
+find what you are looking for.
+
+.. toctree::
+
+   ../../genindex
+   ../../coq-cmdindex
+   ../../coq-tacindex
+   ../../coq-optindex
+   ../../coq-exnindex
+
+For reference, here are direct links to the documentation of:
+
+- :ref:`flags, options and tables <flags-options-tables>`;
+- controlling the display of warning messages with the :opt:`Warnings`
+  option.