polish docs before going public

[seanstory]

part of https://github.com/elastic/enterprise-search-team/issues/4730

Attempt to address some feedback from @leemthompo :

-  Add more context, introduction in the top level introduction to the service:
    - What's it for, when you might want it
    -  It's self-managed with docker
    - use cases
    - version compatibility 
- Probably want to focus on using the service, versus releasing it
- Make sure there's plenty of cross links and context to ingestion mechanisms it was designed to supplement, ie connectors et al
- ideally we'd have a more integrated usage example with a realistic connector workflow

Checklists

Pre-Review Checklist

• [x] this PR has a meaningful title
• [x] this PR links to all relevant github issues that it fixes or partially addresses
• [x] this PR has a thorough description
• [x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)
• [x] Considered corresponding documentation changes