docs: update 'Litmus Uses' to 'Litmus Use Cases' (#350)

[72umesh]

What this PR does / why we need it:

• This PR updates the Use Case documentation to replace Litmus Uses with Litmus Use Cases
  for better clarity and consistency across the documentation.
• Updated title and sidebar_label in website/docs/introduction/usage.md

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #350

Checklist:

• Fixes Update sidebar and title of Use Case doc #350
• Signed the commit for DCO to be passed
• Labelled this PR & related issue with documentation tag

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