Write documentation and integrate into Tivua

[Andreas Stöckel]

Provide a markdown file with high-level documentation.

In particular, there should be a description of the filter language and supported markdown tags.

This document can be linked in the GitHub readme and rendered in the frontend UI when clicking the "Help" menu item.

So really, there are two tasks:

• Write a nice document in markdown
• Implement a client-side view that downloads the documentation and renders it