Explicitly document cache download

> Hi @duffsterlp
>
> yes, the Conan download cache keeps working as a feature in Conan 2.
> It is enabled and configured with conf:
> ```
> core.download:download_cache: To cache Conan artifacts (like conan_package.tgz) downloads
> tools.files.download:download_cache: to cache user (recipes download/get) downloads
> ```
>
> It is true that there is no separate and clear docs page for it, and now it is kind of partially included in https://docs.conan.io/2/devops/backup_sources/sources_backup.html, and mentioned in the conf page https://docs.conan.io/2/reference/config_files/global_conf.html#core-download-download-cache
>
> It might be worth documenting it in the docs in its own page.

_Originally posted by @memsharded in [#3276](https://github.com/conan-io/docs/issues/3276#issuecomment-3208269231)_

cc @duffsterlp

Read the existing backup sources page and the global_conf page linked in the issue to understand the current cache coverage. Create a dedicated documentation page for Conan download caching, covering both configuration entries and their purposes, and ensure the existing references remain consistent.

Written by the indexing model from the issue text.