Add manual pages and content for upcoming GDScript trait system

[Meorge]

The trait system for GDScript being worked on at godotengine/godot#97657 appears to be mostly done, but "Write Documentation" is one of the items still unchecked. The main Godot repository only contains the class reference, which isn't especially applicable to a new language feature.

I think it'll be important for the success of this feature to prepare the user manual to describe traits, so that as soon as they're available in a stable release, users can read about them and understand how to use them.

Edit: A PR has been opened at #10393, and progress on completing the necessary work is now being documented there!

[tetrapod00]

Thanks for trying to document a new feature ahead of time! Often we're catching up and documenting features from the last minor version.

I would just caution you that the PR itself may or may not be close to being approved and merged - I skimmed the PR and while it seems like a lot of work is being done, it's not yet finalized, and it may change in ways that require redoing the documentation later. If that's okay with you, an early documentation draft is very welcome.

A draft PR is perfectly fine, and IMO is preferable. But you can also wait until it's more complete if you want, too.

Your list of potential changes already seems very comprehensive. I do like the idea of an "Introduction to traits" page is good, if you feel there is enough information for one. My guess would be after GDScript: An introduction to dynamic languages?

As for potential missed places, there might be a spot or two in the Best Practices section that makes some implicit assumptions about only classes existing, not traits. But honestly, changing that seems slightly outside of the scope of the initial PR documenting traits.

[Meorge]

Thanks!

I agree, it will definitely be important to ensure this documentation stays up to date with the actual state of the PR itself. I know there's been discussion about whether uses should be the keyword to use, for example; for now, because it's what the test files in the branch use, it's what I've put in the docs so far. My hope is that the structure and rules of the feature are largely complete now, so we may just need to make small modifications to the docs as they come up.

I'll probably move this to a PR shortly. I expect the "Introduction to traits" page will take a fair amount of time and work, and the additions to the style guide will require some community discussion. Perhaps we can hold those discussions on the PR thread, so that they're closer to the branch and diffs.

Thank you for suggesting the Best Practices section! I'll take a look at it soon and see if there are any spots that seem like they'd especially benefit from discussing traits, but otherwise I'll likely leave it out to avoid making this PR's scope too large as you suggested.