Improve documentation around styling and theming

[villesau]

Summary

Looks like documentation around styling and theming is pretty minimal: https://remirror.io/docs/concepts/styling/

Would be great to see styling and theming examples for remirror. Now there's very minimal information scattered in issues, and some of it is misleading or not working. This would greatly help with barrier of entry. I'd love to see styling and theming examples, styling options and pitfalls documented and so on. I'm not able to contribute that at this point due to minimal understanding of the styling capabilities of remirror - if I were, I would've opened a pull request.