Enable `register-docs` by default

For UX reasons, it would be nice to have `register-docs` default-on. Quite a few people have already wondered why their RustDoc comments aren't shown in the editor.

We could disable the registration of docs at runtime if Godot runs in release/exported mode.

This would come at the cost of the (lightweight) `markdown` + `unicode-id` dependencies, which I think is acceptable.

In theory we can still keep the feature and let people opt _out_ of it, however right now we have a setup that mandates several private features like `__codegen-full` to be provided. Maybe there's another way to set this up for CI and `check.sh`, e.g. through a command-line `cfg`.