Changing version on a not-found page redirects to the 404 page for that version instead of that version's page.

**Issue description:**
I was having an issue implementing something in godot 3, and was reading a thread about it. I clicked a link someone gave to documentation about rigid bodies ( https://docs.godotengine.org/en/stable/classes/class_rigidbody.html? )

This class page no longer exists in godot 4's documentation , so I got a '404' page, which said
"Sorry, we couldn't find that page. It may have been renamed or removed in the version of the documentation you're currently browsing."

So I clicked on the 'version' thing to try to change versions to godot 3.

Unfortunately, the version change links for 'not found' pages don't link to the correct place - if I click '3.6', it should link to https://docs.godotengine.org/en/3.6/classes/class_rigidbody.html . Instead, the links take you to the 404 page for that version ( https://docs.godotengine.org/en/3.6/404.html ). This is very unhelpful.


This should be changed, to make it easier to access v3 documentation links which had 'stable' in them when they were given.