Move API content to the new toplevel API section, link the TopNav and light it up in the search

[borland]

Our new UI design intends for all the API-related docs to go under /docs/api, and be accessible via an "API" button in the top-level nav bar. The API docs also include generated docs, synced from the Octopus Server repository on a regular basis.

These generated docs were already present but not linked anywhere. This PR finishes the job, as follows

• The top-nav "API" button now goes to /docs/api

  • The API subsection of the site includes the generated docs, and has it's own special navigation menu which is aware of the API structure, with different UI layouts.

• Pages that were previously inside the /docs/octopus-rest-api folder have now moved to /docs/api

• Fixed all the links from /docs/octopus-rest-api/** to now go to /docs/api/**

• Redirects have been installed for /docs/octopus-rest-api so external links pointing to the old URL's should still work

• The "under construction" code has been removed, so the pagefind search index should now include generated API docs. Including change to the "Facet" thing

Reducing Risk

I've tested locally, but will re-test properly on the staging site (will check off as I've done)

• TopNav goes to api
• TopNav API section and other sections hilight appropriately
• Scroll-to-visible fix in the nav works across all sections (shaun recently implemented this, just checking I haven't re-broken it)
• Generated content from the API docs appears in the search

Hit some old octopus-rest-api/ links and check they redirect to the correct new URL

• docs/octopus-rest-api/octopus.client
• docs/octopus-rest-api/examples/artifacts
• docs/octopus-rest-api/openid-connect

Redirects are annoying to test on the staging site as the redirects hardcode links to octopus.com/docs which won't be valid until this PR merges. But if you take the /docs/api/...whatever part of the URL from the redirect and check it on the staging site, it's good

Please bash at it and do whatever you think might be worth testing

[team-marketing-branch-protections]

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