Fix #17125

Fix tether typo

One extra `</p>` was out of place, plus couple of issues with `'s which caused some odd formatting.

rewrite navbar once more for lower key styles and a new take on modifiers for improved customization

- Remove nested > selectors for dropdown items
- Update docs examples to use all the new markup

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color

- Fixes #13267 somewhat.
- Instead of a single, block-level class, let's use a combination of existing elements and classes.
- For block-level help text, use p.text-muted.
- For inline-level help text, use span.text-muted or small.text-muted.