Update REST API doc structure

[rachaelshaw]

Goal

User story
As a user of docs,
I want to be able to more easily navigate Fleet's API.

How

• Display only the most important API sections in the left-side nav:

  • Authentication
  • Hosts
  • Software
  • Queries
  • OS settings
  • Policies
  • Scripts
  • Vulnerabilities (maybe)
  • Activities (last/maybe)

• Remove the following from the left-side nav:

  • Debug
  • Translator
  • Setup experience
  • Sessions
  • Integrations
  • Fleet configuration
  • File carving
  • Teams
  • Labels

• @mike-j-thomas: Create a simple landing page for /docs/api.

  • Figure out how to access the endpoints removed from the left navigation in the point above.
  • Add an intro section about the API.
  • Keep the API errors content on there.

• @eashaw: Figure out how to achieve this new navigation without splitting the docs into single pages.

• Rachael started to tackle some of this on the api-docs-experiment branch and demoed in this Loom video.

---

Other things to think about, not necessarily as part of this PR.

• Fill in intro paragraphs under for some REST API pages (not every section has one).
• Give the side nav a max-width so we don't have this issue with the edit button:
  
• Be aware of the right-side navigation. Might need some fiddling for longer docs pages. Also, there's a padding issue as the masthead and search box scroll out of view.
• Reconcile the updated structure with outstanding documentation PRs.

[rachaelshaw]

@mike-j-thomas @eashaw @Sampfluger88 I am happy to take on this work btw; I have capacity this sprint.

[mike-j-thomas]

Thanks, @rachaelshaw . Happy if you can take this on. I'll check out your Loom and I also have designs drafted in the backlog that I can share with you. Just haven't been able to get capacity to implement.

[mike-j-thomas]

@rachaelshaw, I have the go-ahead to prioritize this and will draft up what was discussed in design review with Mike M. I have reshaped the issue description to reflect this. Also, it sounds like your focus for the sprint has shifted, so I will likely assign it to @eashaw when he is free.