Document descriptions for doc groups

[lud]

First attempt at the docs. Docs are hard, as always.

I splitted the different concepts (attaching to groups / descriptions / ordering) for clarity, not sure if it is OK.

We may want to fix default ordering (displaying groups from :default_group_for_doc before Types,Functions,Callbacks) and update the docs before merging this.

[github-actions]

📦 Docs artifacts are ready: https://github.com/elixir-lang/ex_doc/actions/runs/15731616733/artifacts/3353705068

[josevalim]

We may want to fix default ordering (displaying groups from :default_group_for_doc before Types,Functions,Callbacks) and update the docs before merging this.

Are you planning to send a PR for this too? :) You can also amend this one if you prefer!

[josevalim]

Suggested change

	Descriptions can be given in the `:default_group_for_doc` configuration:
	Descriptions can also be given in the `:default_group_for_doc` configuration:

[lud]

Are you planning to send a PR for this too? :) You can also amend this one if you prefer!

If we have to fix it then yes I'll update this PR.

[josevalim]

Yes, I should probably preserve it to avoid breaking changes, unless it would add a lot of complexity or complications. I am hoping it is rather just swapping operands somewhere :D

[lud]

Yeah ok, it should not be that hard.