Add instructions on how to setup compiledb for GDextension

**Your Godot version:** 4.4.1

**Issue description:** Currently, the only way to find out how to get compiledb is to dig through github issues, since no place in docs mentions the actual setup of compiledb, it is only expained in the section of vscode, which is not intuitive.
This could be easily solved by adding a mention of the compiledb=yes flag to the GDextension introduction