Add app documentation

[jramboz]

Really needs full docs, but this seems to be the most important.

[jramboz]

After some research, I think I'm going to use Material for MkDocs to create the help files, and then just display them inside a web browser window. The native Qt help format requires the help file viewer to be installed natively on the host machine, and it doesn't seem like you can package it portably. So going with Markdown -> HTML seems to be the most portable and quickest to implement.

Now I just have to write them 😅