[Meta] [Serverless] Update standalone agent docs based on reference yaml

[kilfoyle]

Update standalone agent settings docs based on current elastic-agent-reference.yml.

Structure of a config file will provide a skeleton linking to each of the above sections.

Sections to fix up:

• input Expand standalone agent input settings docs with example, list of types #556
• providers
• output
• ssl/tls
• logging
• feature flags

Section to add:

• agent_download

[kilfoyle]

Closing. I think it's mainly the input settings that needed updating. I'll revisit this if other sections need fixing up as well.