Add docs for certificates flow between components

[kilfoyle]

We need an overview of the connections / certifications flow between components when either simple one-way or mutual TLS is configured for Elastic Agent, Fleet Server, and Elasticsearch

See for reference: https://docs.google.com/document/d/1y-uXR53EkfTbDSNB1Fj4i2WQzD7n57DLQmvRL0VDZ84/edit?pli=1