Docs are missing environment variables for setting up fleet & agent in a container

[kilfoyle]

As noted here the Elastic Agent environment variables page is out of date:

KIBANA_FLEET_SETUP has been removed for a while: elastic/elastic-agent#2910

We're missing some env vars in our public docs (as well as from the output of the container help command):

• FLEET_FORCE - same as the --force flag
• ELASTIC_AGENT_CERT & ELASTIC_AGENT_CERT_KEY - paths to mtls cert and key the agent will use to connect to fleet-server. (corresponding var for passphrase is missing)
• FLEET_SERVER_CLIENT_AUTH - set fleet-server mtls settings, one of: none (default), optional, required
• FLEET_SERVER_ES_CERT & FLEET_SERVER_ES_CERT_KEY - fleet-server -> es mTLS cert paths. (corresponding var for passphrase is missing)
• FLEET_SERVER_INSECURE_HTTP - expose fleet-server as an http server
• FLEET_HEADER - I think this one is headers the agent will send to fleet-server
• FLEET_KIBANA_HEADER - headers used when contacting Kibana
• FLEET_SERVER_TIMEOUT - how long elastic-agent will wait for fleet-server to checkin as healthy
• FLEET_DAEMON_TIMEOUT - how long fleet-server will wait during the bootstrap process for elastic-agent