New navigation

[kmoscoe]

This PR implements the navigation proposed at https://docs.google.com/document/d/1tp2hLKSJXgSFJcED3N40WCMbEOoj8MfZy4mtj6JvsWM/edit?resourcekey=0-OX5FrCW7cb0RN3xKUsg3sg&, using the existing front-matter and sidebar processing methods. In particular, it makes the following changes:

• The "how to use" page becomes the default landing page (index.md)
• The "what is" page (formerly index.md becomes what_is.md
• The standalone pages are now under this top-level heading
• Sheets and Web Components are pulled out from under "API" and become top-level headings
• All headings use a consistent task-oriented style
• Subpages of datasets are removed from nav
• Redundant headings (e.g. "Blog") are removed from nav
• "Import datasets", which can in no way be used by clients, is unpublished now

Staged at bullie.svl.corp.google.com:4000

Still to be handled: "Courseware".

[beets]

sorry for the late review on this! just one comment on keeping the pdf of the papers here, but otherwise lgtm :)