Re-organizing API documentation collections (#169)

* Re-organizing API documentation collections

These changes make it a bit more manageable to collect our various
API specifications into a single navbar item in our docusaurus site.
With the upcoming addition of the Hubble information, this will
ease the implementation, and make things a bit more uniform.

- I've included some nginx rewrites that I _think_ will do the
  correct redirections from any old URLs to the new ones.
- There is now a single `API` dropdown menu item in the site's
  navbar, instead of having separate navbar links for all the
  various APIs, we can now collect them under a single button.
- The `DocSidebar` component has been swizzled to allow us to keep
  the "category" of API used in the breadcrumbs, but to not
  clutter the sidebar with extra (and unnecessary) links.

* Some updates to the anchor-platform openapi output
* Fixing some broken links
* Fixing the `/api/horizon` redirect with a negative lookahead. got myself into a redirect loop there.
* Including `anchor-platform` in the negative lookahead
* Changind dropdown menu text `API` -> `APIs`

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Signed-off-by: Elliot Voris <[email protected]>