Document the HF_DATASETS_CACHE environment variable in the datasets cache documentation

[Harry-Yang0518]

This pull request updates the Datasets documentation to include the HF_DATASETS_CACHE environment variable. While the current documentation only mentions HF_HOME for overriding the default cache directory, HF_DATASETS_CACHE is also a supported and useful option for specifying a custom cache location for datasets stored in Arrow format.

This addition is based on the discussion in (#7457), where users noted the absence of this variable in the documentation despite its functionality. The update adds a new section to cache.mdx that explains how to use HF_DATASETS_CACHE with an example.

This change aims to improve clarity and help users better manage their cache directories when working in shared environments or with limited local storage.

Closes #7457.