a11y: Update pull request template with accessibility checks

Refine items in PR template. Start rigorously checking PR template when proposing and reviewing changes.

## Accessibility

### Images

- [ ] [has alt text](https://www.wcag.com/blog/good-alt-text-bad-alt-text-making-your-content-perceivable/)
- [ ] has transcription of text content
- [ ] [has sufficient contrast ratio](https://webaim.org/resources/contrastchecker/)
- [ ] numeric labels in keyboard nav order
- [ ] description of items with numeric labels in keyboard nav order

### Terminology

- [ ] abbreviations defined where introduced
- [ ] technical terms and jargon defined where introduced
- [ ] add new technical terms, jargon, and abbreviations to glossary
- [ ] [branding terms](https://docs.rc.uab.edu/contributing/contributor_guide#terminology) defined where introduced

## Style Checklist

### References

- [ ] crosslinked from my content to other relevant content
- [ ] crosslinked from other relevant content to my content
- [ ] crosslinked to glossary terms

### Site Navigation

- [ ] `mkdocs.yml` redirects for header and page name changes

Start by locating the repository's pull request template and compare it with the requested Accessibility, Terminology, References, and Site Navigation checklist items. Check the referenced mkdocs.yml redirect guidance as needed. Done means the template includes the requested checks and links, with no unrelated changes.