[Request] Document expectations for deleting files in an agent's "downloads" directory

## Description

*(Describe what needs to be documented. Link to relevant issues.)*
Document our policy for managing/deleting the files that reside in a Elastic Agent's `downloads` directory (on a Standalone Elatic Agent this can be found at `[install_dir]/elastic-agent-8.3.3-darwin-aarch64/data/elastic-agent-xxxxxxx/downloads` for example, but may vary based on OS). Over time the `downloads` directory beneath an Elastic Agent will accumulate with various files for installing Integrations, similar to the list below:

Based on discussions with Elastic Agent developers, it is safe to delete the contents of the `downloads` directory to free up space, as long as the `downloads` directory itself is not deleted, and when deleting the `tar.gz` files, the `asc` and `sha512` files that go with them are also removed. If a package is deleted but later needed by the Agent, it should be able to fetch it again freshly.

## Collaboration
*(Choose the expected collaboration model and delete the others.)*

- The docs team will lead producing the content

**Contact Person:** @dedemorton

## Suggested Target Release

N/A

## Stakeholders

N/A