[IMP] doc: improve organization of upgrades docs

* Add autodoc directives to generate documentation directly from
  upgrade-util repo.
* Group all upgrades-related reference documentation into one group in
  the sidebar, instead of having two entries.
* Mention `upgrades` vs `migrations` directory. It has been supported
  since a while but nothing was explicit in the documentation.

closes odoo/documentation#8748

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>

content/developer/reference.rst

@@ -12,7 +12,6 @@ Reference
     reference/user_interface
     reference/standard_modules
     reference/cli
-    reference/upgrade_scripts
-    reference/upgrade_utils
+    reference/upgrades
     reference/external_api
     reference/extract_api