Helgobox: Add instructions for manual installation

[helgoboss]

For manual installation, it's important to download the app archive and place it at the correct position. There are also toolbar icon files that should be installed. It would be good to document this somewhere.

One generic way would be to let the ReaBoot website dynamically generate the instructions - for all packages. This is a bit challenging though because we would need to download and parse the repository index in the browser.

So maybe just add it to the Helgobox page download section.