[Docs] Add support for Elastic-managed connectors in Observability and Security solutions

[meghanmurphy1]

Bug Description

The docs currently say that Elastic-managed connectors are not supported at all in ECH. They should be updated to say Elastic-managed connectors are supported in Observability and Security Solutions on ECH

Expected behavior

Docs say that Elastic-managed connectors are supported in ECH for Observability and Security solutions