[Quick Accent] Activation method and language list updates

### Describe the requested doc changes

We've made a few changes to the application since the [Quick Accent](https://learn.microsoft.com/en-us/windows/powertoys/quick-accent) page was last updated:

1. Clint added the cool "Hold to activate" functionality, so the "How to activate" section will need an update.
2. New languages have been added to the existing sets, so the "Character sets" section is outdated. (This is less of a priority, as there is a link to `CharacterMappings.cs` which will always contain the full list.)

Note to self: there's also #46595 about the "Start selection from the left" option being missing from the doc.