on-screen-keyboard: Add some clarifications based on feedback

This makes the following changes:

- Rewords some parts for clarity
- Mentions the Mildenhall widget and why it cannot trivially be
  repurposed
- Moves up some information on why Weston is not suitable as-is and
  links down to the "Recommendations" section, to serve as a highlight.

https://phabricator.apertis.org/T8305



Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>