docs: remove outdated information on sync notes page

[augustoschnorr]

Closes #869

• Updates /sync-notes to state that Google Drive is not natively supported on Android
• Also remove Android from Syncthing Recommended systems since the app has been discontinued

[sigrunixia]

Looks like a Syncthing fork is still around. https://github.com/Catfriend1/syncthing-android

I plan on merging this after 18.10 release, with some updates, as we have made some changes to the android app to allow sandboxing and that will change how syncing works depending if they are using app storage, or device storage.

[augustoschnorr]

About the Syncthing, yes there's a fork, but it is not Syncthing anymore, so we can refer it on the docs as a separate option. I haven't added it because I thought it would be misleading to add a link to an unofficial fork of the project.

About the release of 18.10, I saw it was released last week, but I didn't understand how it impacts the sync. Anyway, feel free to edit on top of this. Hope that kick off helped in some way.