docs: add docker guide, and improve index

[pataar]

This adds a guide for installing Mainsail through Docker, and improves the index.md just like the README.md from the repository.

[lixxbox]

I would stick with "NGINX". That's how it's spelled on their website https://www.nginx.com and that's how i included it in the spell checker and changed it in the rest of the docs.

edit: I may be wrong here: nginx.org

[pataar]

@lixxbox @prcutler Feedback fixed!