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.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[lhoestq]

Your clarification in your comment at #7480 (comment) sounds great, would you like to update this PR to include it ?

[Harry-Yang0518]

Hi @lhoestq, I’ve updated the documentation to reflect the clarifications discussed in #7480. Let me know if anything else is needed!

[lhoestq]

Thanks !

I also too the liberty to remove unnecessary \