Improve synchronisation set-up UX

Checklist

• I have used the search function for open and closed issues to see if someone else has already submitted the same feature request.
• I will describe the problem with as much detail as possible.
• This request contains only one single feature, not a list of multiple (related) features.

App version

2.7.0

Where did you get the app from

Google Play

Problem you may be having, or feature you want

Works well enough, but synchronizing between devices with gpodder is completely broken, and documentation is non-existent. When I try to set it up, it gives me a blank text box and wants me to enter "Host name". What does that even mean?? No documentation..

https://play.google.com/apps/publish?account=8008695526664634386#ReviewDetailsPlace:p=de.danoeh.antennapod&reviewid=3a5cfe15-4392-406f-b882-6fef6e040bd8

Suggested solution

• In the 'Choose your synchronisation provider' dialog (SynchronizationPreferencesFragment.java / alertdialog_sync_provider_chooser.xml), above the list of providers:
  • add the following generic intro text: Devices need a central server to synchronize. AntennaPod does not offer such central synchronization server, but allows you to connect with the different types of servers listed below.
  • add a text link to open relevant documentation with an open in new icon and the label/link text More information. The link should open /s/sync-help prepended with site language code if available for device language. For localisation of links see the following: https://github.com/AntennaPod/AntennaPod/blob/261c7982defc86607a45acaba01c913abe0e6582/app/src/main/java/de/danoeh/antennapod/fragment/preferences/MainPreferencesFragment.java#L144
• In the second and subsequent steps of the gPodder process, add a way to go back (e.g. to allow to edit the server if you notice a typo when entering credentials). See ViewFlipper in GpodderAuthenticationFragment.java
• In the 'Login' dialog of all providers (currently gPodder & Nextcloud):
  • add a generic intro text: Please provide the address (URL or IP address) of your synchronization server.
  • change the label 'Host name' to 'Server address'

Screenshots / Drawings / Technical details

No response