Added description of core development priorities to Product Vision

[gerald-hartig]

Link to issue number:

n/a

Summary of the issue:

Update to product vision to clarify which features NVDA supports in core.

Description of user facing changes

Addition of text to Product Vision -> Product description

Description of development approach

n/a

Testing strategy:

n/a

Known issues with pull request:

None

Code Review Checklist:

• [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.