Add "Upgrading from Godot 4.2 to Godot 4.3" page

[raulsntos]

It's that time of the year again. This documents all the breaking changes in 4.3, like we did for 4.2 (https://github.com/godotengine/godot-docs/pull/8488), and 4.1 (https://github.com/godotengine/godot-docs/pull/7611).

Area maintainers should review the changes listed for their area. Also feel free to suggest any addition that I might have missed or make follow-up PRs.

Updated as of commit godotengine/godot@214968243c442c0fdcbdfef23943c1547aeafdbc