docs: standardize book titles to === with overline

Documents within a Sphinx manual are separate files and therefore can use
different conventions for headings.  However, keeping some consistency is
useful so that included files are easy to get right.

This patch uses a standard heading format for book titles, so that it is
obvious when a file sits at the top level toctree of a book or man page.
The heading is irrelevant for man pages, but keep it consistent as well.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>