Add documentation on how the cache works

## What New Or Updated Would You Like To See?

Based on this [discussion](https://github.com/chocolatey/choco/discussions/2698) it would be good to add a technical explanation on how the cache works.

## Why Is It Needed?

There is some confusion about what the cache is, what it does and why.

## Additional Context?

See [this discussion](https://github.com/chocolatey/choco/discussions/2698).

Start by reading the linked discussion at chocolatey/choco#2698 to understand the cache terminology and the source of the confusion. Review the documentation structure to choose the appropriate location, then add a technical explanation covering what the cache is, what it does, and why it exists. Done means the explanation is published and addresses the questions raised in the discussion.

Written by the indexing model from the issue text.