feature: Add manifest fetch documentation for digest

[TerryHowe]

What this PR does / why we need it:

This PR adds an example for --format and it is a common task (for me at least) to get the digest.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.49%. Comparing base (f2affdd) to head (48b3a47).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1465   +/-   ##
=======================================
  Coverage   85.48%   85.49%           
=======================================
  Files         109      109           
  Lines        3851     3853    +2     
=======================================
+ Hits         3292     3294    +2     
  Misses        335      335           
  Partials      224      224           

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[sajayantony]

Funny I do the same thing instead of resolve https://oras.land/docs/commands/oras_resolve