docs: add initial documentation to external assets

[cpvalente]

Feedback from @RasterboxOff:

• the usage of external folders becomes more visible with the new menu and we would like to provide users a way to find out what they do

Also adding some small comments to the CSS properties

@alex-Arc , before we expose this, it would be a good change to review the structure
we plan on adding a sound file and a user given logo to the app
The sound file is part of Ontime
The logo belongs to the user

I wonder if the structure should be

my-app/
├─ projects/
├─ external/
│  ├─ demo/
├─ user/
│  ├─ styles/
│  │  ├─ override.css
│  ├─ sound/
│  │  ├─ alarm.mp3
│  ├─ logo/
│  │  ├─ logo.png```