docs/sphinx: change default role to "any"

docs/sphinx: change default role to "any"

This interprets single-backtick syntax in all of our Sphinx docs as a
cross-reference to *something*, including Python symbols.

From here on out, new uses of `backticks` will cause a build failure if
the target cannot be referenced.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20211004215238.1523082-4-jsnow@redhat.com>