Fixes #18505
Closes #18509

Refs #18110

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Refs #18459, #17178
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See a8dc4812#commitcomment-14677726

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[V4 Docs] Update migration docs to point out the .btn-default replacement.

V4 Docs - Updates to migration docs to pointed out the removal of .btn-default class and an addition of the new .btn-secondary class.

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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