Closes #17291 by finishing the last remaining task.
Also reorders the sections in the "Download" docs.

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[skip sauce]

Closes #17214.

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Mostly links.
Fixes #17056.

- 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