Explicitly document cache download

[AbrilRBS]

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

cc @duffsterlp

[duffsterlp]

Thanks @AbrilRBS and @memsharded for creating this separate issue.