Improve documentation on how to create an extension

[villesau]

Summary

Now the documentation only lists what extensions are possible: https://remirror.io/docs/advanced/creating-extensions

It would make sense to have a step by step guide to create a simple extension too. It could go through crucial methods and the purpose decorators etc. Now when copying some simple extension to editor, all the decorators tend to give plenty of type errors which makes them harder to grasp.

I believe that by fixing docs around extension could help to get external contributors to reduce pressure from core maintainers as more people could fix bugs and improve the framework.