gdext: API and code style guidelines

Write a page about style-related topics that cannot easily be automated and covered by rustfmt/clippy.

APIs:
* proc-macro attribute structure
* redundancy / when to use multiple methods
* `#[must_use]`

Implementation:
* order of symbols within a file
* order of `#[derive(...)]` traits
* `use` statements

This issue serves as a collection of more topics, as they come up in PR reviews.