Move Observability products under Observability heading in Kibana reference

[bmorelli25]

The Kibana writers would like us to move the Observability products under the Observability heading in the Kibana reference. The APM content cannot be moved, but Logs, Metrics, and Uptime can. This move will help up clean up their long TOC, without requiring us to refactor any APM content.

The idea discussed would be to have all Obseravability solutions on one page, with links to the docs:

• Observability
  • Observability overview
  • APM (link to kib ref)
  • Logs (link to obs docs)
  • Metrics (link to obs docs)
  • Uptime (link to obs docs)

The new Kibana TOC would then look something like this:

• ...
• Graph
• Visualize
• Observability
• APM
• Elastic Security
• ...