Update shortcut documentation for recent releases?

[ferdnyc]

The app currently provides instructions for setting up a TalkBack shortcut via the Accessibility shortcuts settings:

https://github.com/appt-org/screenreader-android/blob/main/app%2Fsrc%2Fmain%2Fres%2Fvalues%2Fstrings.xml#L58-L61

While those instructions do still work, recent Android releases have dedicated TalkBack settings that may provide clearer explanations and additional options.

If the user instead goes to Settings > Accessibility > TalkBack > TalkBack shortcut, they'll find not only a switch to enable/disable a shortcut, but they can tap on the setting item to view a dedicated menu listing all of the shortcut assignment options.

For sighted users only using TalkBack for testing purposes, placing the shortcut on the navigation bar ("Accessibility button") may be more convenient than assigning a hardware button shortcut.

Perhaps the app should either recommend that option, instead of the volume keys, or at least note that there are several options for what shortcut to assign?