Document cloudbeat functional dependency on beats repo

**Motivation**
Cloudbeat depends on [beats](https://github.com/elastic/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` | ... |