Fix documentation errors: remove misplaced examples and clean up formatting

Validated and fixed multiple sections with erroneous content:

- Organizations: Removed wrong fields API and opportunities examples
  - Removed 'Penny Opportunity' and 'Affinity Opportunity' (belong in Opportunities)
  - Removed /organizations/fields example (wrong API endpoint)
  - Added correct organization search examples with proper structure

- Interactions: Removed stray DELETE /notes example from Get Specific Interaction

- Entity Files: Removed webhook JSON examples from Get All Entity Files section

- Reminders: Removed duplicate 'Delete a Reminder' heading with wrong PUT request

- Changelog: Fixed formatting issues
  - Removed stray text fragments after entries
  - Added proper markdown link for 'creating a note' reference

All changes validated against live API documentation at https://api-docs.affinity.co/
Total: 357 lines removed, 26 lines added (-331 net lines of incorrect content)
No regressions - all fixes improve accuracy and consistency with official docs.

Author: yaniv-golan