User guide: MathCAT edits (#19234)
### Link to issue number:
None
### Summary of the issue:
Edited the MathCAT section of the user guide, both the overview and
settings sections for clarity and organization.
### Description of user facing changes:
User guide content edits.
### Description of developer facing changes:
The following edits were made:
* Typical use is now a level 4 heading
* ETS acronym expanded (Educational Testing Service)
### Description of development approach:
Edited and proofread to make sure content is clearer.
### Testing strategy:
None (manual proofreading is recommended)
### Known issues with pull request:
None
### Code Review Checklist:
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- [x] Documentation:
- Change log entry
- User Documentation
- Developer / Technical Documentation
- Context sensitive help for GUI changes
- [x] Testing:
- Unit tests
- System (end to end) tests
- Manual testing
- [x] UX of all users considered:
- Speech
- Braille
- Low Vision
- Different web browsers
- Localization in other languages / culture than English
- [x] API is compatible with existing add-ons.
- [x] Security precautions taken.
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Co-authored-by: Sean Budd <seanbudd123@gmail.com>
