Document cloudbeat functional dependency on beats repo

[moukoublen]

Motivation Cloudbeat depends on beats module/repo.

Many things that construct a beater are implemented in standalone elastic-agent-* modules (e.g. elastic-agent-libs, elastic-agent-client etc).

We could document the functional parts from beats that Cloudbeat depends on and are not implemented in standalone modules.

For example, Beat cmd (github.com/elastic/beats/v7/libbeat/cmd) implements the CLI handling, which is not implemented in the standalone modules.

Definition of done A document with the functionality implemented in beats code (and not in standalone modules).

Example:

Title	Sub-Package	Description
Mage files	dev-tools	mage files functions that implement the build life-cycle ...
CLI	libbeat/cmd	...