[DOC] doc: add documentation for mixins and useful classes

Documentation to help developers use the following classes:
- utm.mixin
- rating.mixin
- website.published.mixin
- website.seo.metadata
- mail.thread
- mail.alias.mixin

After careful consideration, the ir.needaction.mixin was
not included in this doc page since it is no longer displayed
anywhere and will be dropped quite soon.

doc/reference.rst

@@ -20,5 +20,6 @@ Reference
     reference/translations
     reference/reports
     reference/workflows
+    reference/mixins
     reference/guidelines
     reference/upgrade_api