Enhance markdown for ssl configuration in elastic-agent

[khushijain21]

This page contains ssl configuration for both server and client side - but it is not clearly seperated into two headings

One can be easily confused on why we have certificate_authorities defined twice until they read the description. I think server and client configuration should be bifurcated into their sub-headings.

Reference can be taken from filebeat->output-> ssl page https://www.elastic.co/docs/reference/beats/filebeat/configuration-ssl

This is much more easily navigable than the elastic-agent docs