docs: Document that get_starting_timestamp requires setting a non-null replication_key

[edgarrmondragon]

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📚 Documentation preview 📚: https://meltano-sdk--2557.org.readthedocs.build/en/2557/

[codspeed-hq]

CodSpeed Performance Report

Merging #2557 will not alter performance

_{Comparing 320-docs-document-that-get_starting_timestamp-requires-setting-a-non-null-replication_key (66c8046) with main (4cafb6f)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.46%. Comparing base (4cafb6f) to head (66c8046).
Report is 133 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2557   +/-   ##
=======================================
  Coverage   89.46%   89.46%           
=======================================
  Files          58       58           
  Lines        4795     4795           
  Branches      937      937           
=======================================
  Hits         4290     4290           
  Misses        352      352           
  Partials      153      153           

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