GDB pretty-printers were recently added for a couple Godot types.
They make the GDB debugger variable display more readable, so let's
document their existence for developers.
cf. godot/godot#91280
The Introduction to the buildsystem page still mentions this parameter
in case it needs to be changed, but in most situations, the default
value of `(number of CPU threads) - 1` does a good job.
- Merge Community/Contributing and Engine Development into one top-level section, Contributing.
- Structure it in a way that gives equal priority to various ways to contribute, including documentation.
- Rename several articles to be more comprehensive and in line with the overall state of the documentation.
- Rewrite the guide for class reference contributions.
