[Request] Improve docs for self-managed HTTPS configuration for Fleet and Elastic Agent

## Description

A few improvements that came out of a recent SDH to help customers configure Elastic Agent and Fleet Server in a self-managed environment using self-signed certificates:
- Add a section to [ES basic security doc](https://www.elastic.co/guide/en/elasticsearch/reference/master/security-basic-setup-https.html#configure-beats-security) about configuring Fleet Server's connection to ES using a self-signed cert
- Maybe just a link to https://www.elastic.co/guide/en/fleet/master/secure-connections.html is sufficient?
- Add CLI options for configuring self-signed certs for Elastic Agent in container examples:
- Essentially add `-e CERTIFICATE_AUTHORITIES`, `-e FLEET_SERVER_ELASTICSEARCH_CA`, `-e FLEET_CA`, `-e FLEET_SERVER_CERT`, `-e FLEET_SERVER_CERT_KEY` options
- @elastic/elastic-agent-control-plane team would know better than me which options are necessary, I think there is some duplication in these flags
- https://www.elastic.co/guide/en/fleet/current/elastic-agent-container.html#agent-in-container-self
- https://www.elastic.co/guide/en/fleet/current/elastic-agent-container.html#agent-in-container-docker