docs: Add explanation and recommendations for context usage

[edgarrmondragon]

From the conversation in #1028 and other conversations in Slack regarding manually adding properties to the context object, it's probably not too clear what can and can't be done with the object at this point.

In the long term we may want to enforce the latter by making the context immutable during stream sync.

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📚 Documentation preview 📚: https://meltano-sdk--1060.org.readthedocs.build/en/1060/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.13%. Comparing base (b9f693b) to head (f87ac25).
Report is 1078 commits behind head on main.

Additional details and impacted files

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  Coverage   83.13%   83.13%           
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  Files          39       39           
  Lines        3747     3747           
  Branches      628      628           
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  Hits         3115     3115           
  Misses        470      470           
  Partials      162      162           

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