[DOCS]: Add some feature and usage docs/readme

I have this app not running in my nextcloud setup, and I probably won't right now. While it sounds interesting, I can't find any offical documentation besides some basic setup instructions and a really barebone feature list (e.g.
📊 Easily add mermaid diagrams - wow, that sounds super interesting... but what does it mean? how does it work?).

I would recommend to add some more description (and images) to the Nextcloud App Store Page, and have more extensive Readme in the repo:
Give me all the features and what they can do - ideally with some images.
Provide some instructions on how to use the app - that would give me a better first impression what the app is about and if it is what I'm looking for.

Sorry if the first line sounds harsh - I just wanted to provide my journey which lead me to write this issue: I think the app is great and I'm a big fan of nextcloud and the ecosystem. But for it to mature even more, things like this should be adressed (imo) - that would catch way more people as it lowers the entry barrier.

Start by reviewing the repository README and the Nextcloud App Store page, then inventory the app's existing features and usage paths. Done means the README and store listing explain the features, include usage instructions, and add helpful images for prospective users.