docs: Refactor merge sort code example to use literalinclude

[davidlin20dev]

Why are the changes needed?

Since I recently added the merge sort code to advanced_composition/dynamic_workflow.py, it makes sense to just reference it using literalinclude instead of copying the code into the docs.

What changes were proposed in this pull request?

Please refer to the Files changed.

Before: Raw Python code included directly in docs
After: Using literalinclude to reference code from flytesnacks

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

flyteorg/flytesnacks#1771

Docs link

https://docs.flyte.org/en/latest/user_guide/advanced_composition/dynamic_workflows.html#merge-sort

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.09%. Comparing base (9dc21f4) to head (9168ce3).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6091      +/-   ##
==========================================
- Coverage   37.10%   37.09%   -0.01%     
==========================================
  Files        1318     1318              
  Lines      132403   132403              
==========================================
- Hits        49122    49121       -1     
- Misses      79009    79010       +1     
  Partials     4272     4272              

Flag	Coverage Δ
unittests-datacatalog	51.58% <ø> (ø)
unittests-flyteadmin	54.10% <ø> (ø)
unittests-flytecopilot	30.99% <ø> (ø)
unittests-flytectl	62.29% <ø> (ø)
unittests-flyteidl	7.23% <ø> (ø)
unittests-flyteplugins	53.82% <ø> (ø)
unittests-flytepropeller	42.60% <ø> (ø)
unittests-flytestdlib	57.57% <ø> (-0.02%)	⬇️

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[pingsutw]

thank you!