docs/sphinx: Add new qapi-doc Sphinx extension

Some of our documentation is auto-generated from documentation
comments in the JSON schema.

For Sphinx, rather than creating a file to include, the most natural
way to handle this is to have a small custom Sphinx extension which
processes the JSON file and inserts documentation into the rST
file being processed.

This is the same approach that kerneldoc and hxtool use.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-8-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Comment and doc string formatting tweaked, unused method dropped,
a few line breaks tweaked to follow PEP 8 more closely, MAINTAINERS
section QAPI updated]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

MAINTAINERS

@@ -2387,6 +2387,7 @@ F: tests/test-qmp-*.c
 F: tests/test-visitor-serialization.c
 F: scripts/qapi-gen.py
 F: scripts/qapi/*
+F: docs/sphinx/qapidoc.py
 F: docs/devel/qapi*
 T: git https://repo.or.cz/qemu/armbru.git qapi-next
 
@@ -3151,6 +3152,7 @@ M: Peter Maydell <peter.maydell@linaro.org>
 S: Maintained
 F: docs/conf.py
 F: docs/*/conf.py
+F: docs/sphinx/
 
 Miscellaneous
 -------------

docs/conf.py

@@ -52,7 +52,10 @@ except NameError:
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use an absolute path starting from qemu_docdir.
 #
+# Our extensions are in docs/sphinx; the qapidoc extension requires
+# the QAPI modules from scripts/.
 sys.path.insert(0, os.path.join(qemu_docdir, "sphinx"))
+sys.path.insert(0, os.path.join(qemu_docdir, "../scripts"))
 
 
 # -- General configuration ------------------------------------------------
@@ -67,7 +70,7 @@ needs_sphinx = '1.6'
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['kerneldoc', 'qmp_lexer', 'hxtool', 'depfile']
+extensions = ['kerneldoc', 'qmp_lexer', 'hxtool', 'depfile', 'qapidoc']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -241,3 +244,4 @@ texinfo_documents = [
 kerneldoc_bin = os.path.join(qemu_docdir, '../scripts/kernel-doc')
 kerneldoc_srctree = os.path.join(qemu_docdir, '..')
 hxtool_srctree = os.path.join(qemu_docdir, '..')
+qapidoc_srctree = os.path.join(qemu_docdir, '..')