Add 3.X <-> 4.0 version cross-references to classes with changed names

**Your Godot version:**
3.X, 4.0

**Issue description:**
It's often hard to impossible to find a class which has been renamed, merged or removed between Godot 3.X and Godot 4.0 without a diploma in Github archeology.

For example I am porting a Godot 3.X project to Godot4.0 which uses the File class. Godot 4.0 does not seem to have the File class. I want to rewrite my script with the appropriate 4.0 approach/replacement, but I am not able to find what the replacement solution is, despite online searching. Sifting through Github issues to figure out [what happened](https://github.com/godotengine/godot/issues/66241#issuecomment-1254623632) is a very unpleasant user experience to put it mildly.

Ideally, I would be able to just switch from the **_stable_** (3.X) online documentation of the class I am interested in, to the **_latest_** online documentation and it would lead me automatically to the 4.0 replacement class.

Now I know the online docs are limited in functionality, so this might not be possible for whatever reason. But we could definitely have a margin box somewhere on the class page saying:

"This class will be removed/renamed in Godot4 to _ClassName_"

ClassName then links to the latest online docs page of the Godot4 class. Likewise the Godot4.0 replacement class could link back to the Godot 3.X class page with a similar text box, like:

"This class replaces the _ClassName_ in Godot3.X " ... at least for a transition period.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_file.html
https://docs.godotengine.org/en/latest/classes/class_fileaccess.html