Clean up documentation and refactor UI strings (#20361)
Follow-up of #20329
Summary of the issue:
Various issues found in documentation and UI strings.
Some issue found by Copilot in #20329; it would probably have been useful to run copilot on #20261.
Description of user facing changes:
Use "Math" as a category in Input gestures dialog, since MathCAT is not used elsewhere in the UI.
Remove the precise pixel specification of pinch gesture, since not useful at user level
Removed touch navigation justification from the User Guide (it's not a PR)
Removed outdated instruction to disable screen curtain when launching magnifier: it is now automatically disabled (and restored).
Fixed negative squared Z character
Fix formatting in UI or documentation
