docs: Stop calling the top level subsections of our manual 'manuals'

docs: Stop calling the top level subsections of our manual 'manuals'

We merged our previous multiple-manual setup into a single Sphinx
manual, but we left some text in the various index.rst lines that
still calls the top level subsections separate 'manuals'.  Update
them to talk about "this section of the manual" instead, and remove
now-obsolete comments about how the index.rst files are the "top
level page for the 'foo' manual".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210705095547.15790-3-peter.maydell@linaro.org