Docs: Unify readme and https://docs.pslab.io/application/Readme.html

[marcnause]

Describe the issue

https://docs.pslab.io/application/Readme.html Contains links to downloads of the PLSab app for all supported systems. The README.md of the plab-app repo only contains links to the Google Play store and F-Droid.

There should be only one source of truce. That's why I am proposing that we use https://docs.pslab.io/application/Readme.html to list all the places where the different binaries can be downloaded and keep the section short here, maybe mention all target platforms, but reference https://docs.pslab.io/application/Readme.html for more details.