Improve organization of Fleet and Elastic Agent Guide (fka Fleet User Guide)

[dedemorton]

The Fleet User Guide has grown organically, and the product design has evolved quite a lot since the beginning. We have a lot of good content, but there are some content gaps, and the existing structure is not ideal. We need to take some time to review the existing content holistically and improve the overall quality and organization.

Tasks:

MVP (target 7.16):

• Create initial outline
• Rearrange existing content
• Add minimum fill content needed to make new structure work
• Move the logs/metrics and traces quick start guides to the Observability guide. PR: Move quick starts to the obs guide #1149
• Set up redirects for all moved topics (based on the redirects.asciidoc page) https://github.com/elastic/observability-docs/pull
• Update URLs in Kibana UI if necessary

Next iteration:

1. Identify content gaps based on review of new structure and existing GitHub issues
2. Make sure each major content gap has a corresponding issue in GitHub
3. Update outline to reflect missing content
4. Create plan to fill in content gaps

[dedemorton]

As part of this effort, we should also make some consistency edits.

[dedemorton]

The first step here is organizing the existing content. I've created a draft outline proposal and will get that reviewed by key stakeholders.

[dedemorton]

The structural changes are now merged in #1109

[dedemorton]

Closing because this work is done for 7.16. I think we still need to identify content gaps, but before we do that, we should get through more of the high priority issues in GitHub.