Calls out the new `.table-inverse`, `.table-reflow`, `.thead-default` and `.thead-inverse`

Audit the new table variants in V4 that aren't in V3

X-Ref: #17120 for `.panel-default` replacement

Example:
> ... SCSS variables (e.g., `@enable-gradients: true`) ...
Referenced SCSS but used the LESS syntax. Changed to:
> ... SCSS variables (e.g., `$enable-gradients: true`) ...

Closes #17114

- 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