Group support for context menus

This allows the user to playback selected text using a group.
Reorder the usage documentation section

This puts the universal options first, then on-the-fly, then presets and
groups, and then finally removing audio.
Documentation forg mass generation and throttling

Explains the per-service handling of throttling and the benefit of a
group.
Update screenshot for Presets configuration

This shows the newer version of the Advanced tab with the separated
buttons.
Update documentation for Advanced config screen

Shows the new Manage Groups button and explains its usage in the
Service Presets and Groups section.
Basic documentation for Groups

Fleshes out the Groups page in the documentation and adds screenshots.