Utilities/Sphinx: Add option to build 'text' format

Add SPHINX_TEXT to enable the Sphinx 'text' builder.  Mark it as
advanced and do not add install rules.  This is intended for use
by the release manager to build the release notes in text format
suitable for email.
Help: Add documents for release notes

Add a release/index.rst document titled "CMake Release Notes" to hold
the toctree for release notes.  Add a "Release Notes" section to the
top-level html document index to link to the new document.
Help: Add documents to collect notes between releases

Add a release/dev.txt file and include it from release/index.rst in
development versions.  Add a "Changes Since Release" section with a
toctree that globs adjacent "dev/*" documents.  Add a sample topic
document explaining how topic-specific release note documents work.

This approach will allow developers to write release notes for their
changes as they are made.  The release manager may then consolidate and
organize the notes for a specific release version.
Merge topic 'release-notes' into next

ccc87047 Help: Add documents to collect notes between releases
70309e70 Help: Add documents for release notes
34ea1f15 Utilities/Sphinx: Add option to build 'text' format