docs: add table of contents to QAPI references

The QAPI reference docs for the guest agent, storage daemon and QMP are
all rather long and hard to navigate unless you already know the name of
the command and can do full text search for it.

A table of contents in each doc will help people locate stuff much more
easily.

Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/interop/qemu-ga-ref.rst

@@ -10,4 +10,7 @@ QEMU Guest Agent Protocol Reference
    TODO: display the QEMU version, both here and in our Sphinx manuals
    more generally.
 
+.. contents::
+   :depth: 3
+
 .. qapi-doc:: qga/qapi-schema.json

docs/interop/qemu-qmp-ref.rst

@@ -10,4 +10,7 @@ QEMU QMP Reference Manual
    TODO: display the QEMU version, both here and in our Sphinx manuals
    more generally.
 
+.. contents::
+   :depth: 3
+
 .. qapi-doc:: qapi/qapi-schema.json

docs/interop/qemu-storage-daemon-qmp-ref.rst

@@ -10,4 +10,7 @@ QEMU Storage Daemon QMP Reference Manual
    TODO: display the QEMU version, both here and in our Sphinx manuals
    more generally.
 
+.. contents::
+   :depth: 3
+
 .. qapi-doc:: storage-daemon/qapi/qapi-schema.json