Document how to find, install and configure OTel integrations for Fleet-managed agents

[vishaangelova]

Create the public-facing documentation on how to find, install, configure OTel integrations for Fleet-managed agents. The MVP feature is planned to be available in 9.2.0.

Out of scope: documentation for building OTel integrations using the otelcol input type.

Resources

https://github.com/elastic/ingest-dev/issues/5613
https://github.com/elastic/ingest-dev/issues/5711

[vishaangelova]

Closing as duplicate of elastic/ingest-docs#1861