Improve class reference split, list variant types separately

[YuriSizov]

Follow-up to #63497 with a number of improvements based on early feedback.

• Fixes the issue with subsection references being duplicated.
• Adds a new main section to serve as a page title, "All classes", which fixes breadcrumbs (always referring to "Globals", no matter which class you were looking at).
• Splits "Objects" into "Other objects" (since nodes and resources are also objects) and "Variant types", giving the latter their own home in the class reference.
• Adds "Object", "Node", and "Resource" as a first entry to their own groups, "Other objects", "Nodes", "Resources"; they also still remain in their current groups, based on inheritance (e.g. you can find "Node" in "Other objects").

PS. I can push a commit with an updated index.rst to the docs repo if this is accepted.

[YuriSizov]

🎉 Pushed godotengine/godot-docs@3cb7181 since this is approved now.

[akien-mga]

Thanks!