Clarify versions and examples in the README

[dsnopek]

This attempts to clarify a few things that have come up in issues recently:

• That users of Godot 4.1 need to use the 4.1 branch. See Can't load test project: Attempt to get non-existent interface function: string_resize #1201
• That the test project isn't meant to be an example anymore. Since PR Add automated tests that run a GDExtension (rather than just building it) #1101 it's only for our automated tests. Instead, link to https://github.com/godotengine/godot-cpp-template and https://github.com/paddy-exe/GDExtensionSummator and the official docs. See demo project missing & README.md hyperlinks to 404 #1217
• Link to both the Godot issue tracker (filtered for GDExtension issues) and the godot-cpp issue tracker for finding/reporting issues. See "Issue tracker" link on godot-cpp README.md actually points to Godot issues #1206

Fixes #1206

[akien-mga]

Thanks!