docs/devel/docs: Document .hx file syntax

docs/devel/docs: Document .hx file syntax

We don't currently document the syntax of .hx files anywhere
except in a few comments at the top of individual .hx files.
We don't even have somewhere in the developer docs where we
could do this.

Add a new files docs/devel/docs.rst which can be a place to
document how our docs build process works. For the moment,
put in only a brief introductory paragraph and the documentation
of the .hx files. We could later add to this file by for
example describing how the QAPI-schema-to-docs process works,
or anything else that developers might need to know about
how to add documentation.

Make the .hx files refer to this doc file, and clean
up their header comments to be more accurate for the
usage in each file and less cut-n-pasted.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc.michel@amd.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Message-id: 20231212162313.1742462-1-peter.maydell@linaro.org