sharded manifests docs

[bmwshop]

What does this PR do ?

Adds documentation on the use of sharded manifests

Collection: ASR

Changelog

• docs/source/asr/datasets.rst changed to add the docs

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

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Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests?
• Did you add or update any necessary documentation?
• Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  • Reviewer: Does the PR have correct import guards for all optional libraries?

PR Type:

• New Feature
• Bugfix
• [ X] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.

Who can review?

Anyone in the community is free to review the PR once the checks have passed.
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Additional Information

• Related to # (issue)

[titu1994]

Thanks !