Redirect stable pages to 3.5 when they appear to be a 3.5 page

**Your Godot version:**
3.5
**Issue description:**
There are a ton of links around the internet that link to the /stable/ docs but are expecting 3.5 pages. They are now all 404 since the 4.0 release.

Search engines:

And of course, Q&A forums:
https://godotengine.org/qa/

Reddit:
https://reddit.com/r/godot/

It would be nice if something could be set up to automatically redirect pages that look like they're meant for a 3.5-specific topic to the /3.5/ URL if they are directing to /stable/.