Add support for user-provided comments with Giscus

This allows users to leave comments on pages that don't have
`:allow_comments: False` somewhere in the page's source.
Both manual and class reference pages can receive comments.
Index pages cannot have comments, as discussion should occur on "leaf" pages.

GitHub Discussions is used as a backend on the same repository. This means
that Discussions *must* be enabled on godotengine/godot-docs before this
commit is merged to `master`. Users can choose to use the "Custom" watch
mode if they don't want to get notifications for discussion updates,
but still get notifications for issue and pull request updates.

User comments are intended to be used for the following purposes:

- Add a clarification or correct something in the documentation,
  without having to open a pull request. Contributors are encouraged to
  take a look at discussions from time to time, and see if there's information
  worth incorporating in the pages themselves. Don't forget to reply to
  the comment when doing so :)
- Mention a workaround for a common issue.
- Link to useful third-party resources that are relevant to the current page,
  such as tutorials or add-ons.

User comments should *not* be used for technical support. Other community
platforms should be used for that.

Page-to-discussion matching is done using the `pagename` Sphinx variable,
which is independent of the Godot version and documentation language.
Being independent of the Godot version allows keeping old comments
when the Godot version changes, while also allowing users from `/stable`
and `/4.1` to "see" each other in discussions.

See https://giscus.app for more information.

tutorials/math/index.rst

@@ -1,3 +1,5 @@
+:allow_comments: False
+
 Math
 ====