[Elastic Agent] Add more developer docs around Elastic Agent

[ruflin]

More and more teams are using and building on top of Elastic Agent. Quite a few things are not documented yet related to developing against the Elastic Agent. This issue is to collect a list of items that should be documented to help other teams and unload the Elastic Agent team from helping others. For now, I would put these docs into https://github.com/elastic/beats/blob/master/x-pack/elastic-agent/README.md

• [ ] Dev setup and how to pull in other binaries like fleet-server

Anyone working on Elastic Agent, please edit this issue and add more items to it.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[ruflin]

@andrewvc @axw and teams, it would be helpful if you could extend this list. @andresrc I expect this issue to grow over time but we should continously work on it as it will save us time in the end.

[botelastic[bot]]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!