Improved documentation

[ueman]

This adds doc pages to the existing API docs. This is similar to the approach of the docs of GoRouter, see for example the following page: https://pub.dev/documentation/go_router/latest/topics/Get%20started-topic.html

This works by making use of categories (also called topics). See dartdoc explanation at https://pub.dev/packages/dartdoc#advanced-features

How to test this

Execute dart doc in the dio folder. This creates the docs. When opening the index.html, you should see the newly added migration page in the API docs.

Further stops

I'll try to migrate further documentation to, but further I'd like some feedback.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

Additional context and info (if any)

Fixes #1921

[kuhnroyal]

Do we need to do anything to deploy this?

[ueman]

Do we need to do anything to deploy this?

Nope. This will be part of the hosted API docs on pub.dev

[kuhnroyal]

Awesome, I ran this locally, looks good!