[Data] Write documentation for users on Kibana Search Sessions

[tsullivan]

Kibana version: 7.12+

This is a tracking issue to mark that user-facing documentation must be written on Kibana Search Sessions.

This will be a new page of Kibana documentation that is relevant to Search Sessions as seen in Kibana.

PR review comment that brought up this issue: https://github.com/elastic/kibana/pull/81707/files#r551031536

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[lizozom]

#89278