FED-1978 Null-safe props documentation

[greglittlefield-wf]

Motivation

We needed some documentation on required props and how they relate to null safety.

We also need to update code blocks in documentation to use null safety.

Changes

• [x] Small README cleanup
  • [x] Make null safety the featured migration guide
  • [x] Add basic docs around function components (can't believe we didn't have any 😓) #boyscouting
• [x] Add required props documentation
• [x] Update code blocks with null safety
  • [x] In README
  • [x] In doc comments

Release Notes

Review

See CONTRIBUTING.md for more details on review types (+1 / QA +1 / +10) and code review process.

Please review:

QA Checklist

• [ ] Tests were updated and provide good coverage of the changeset and other affected code
• [ ] Manual testing was performed if needed
  • [ ] Steps from PR author:
  • [ ] Anything falling under manual testing criteria outlined in CONTRIBUTING.md

Merge Checklist

While we perform many automated checks before auto-merging, some manual checks are needed:

• [ ] A Frontend Frameworks Design member has reviewed these changes
• [ ] There are no unaddressed comments - this check can be automated if reviewers use the "Request Changes" feature
• [ ] For release PRs - Version metadata in Rosie comment is correct

[aviary3-wk]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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Questions or Comments? Reach out on Slack: #support-infosec.

[greglittlefield-wf]

Okay @aaronlademann-wf and @sydneyjodon-wk, I think I've addressed all your feedback!