[Serverless] Create 'structure of a config file' for Standalone Agent

[kilfoyle]

Create a skeletal/structural summary of the Standalone Agent config file.

• Base the topic off of the Logstash Structure of a config file
• Base the list of sections off of elastic-agent-reference.yml (sections should be: inputs, providers, output, ssl, logging, feature flags, agent_download)
• Another helpful reference is the github Elastic Agent policies document
• Make the left pane navigation match the skeleton.