docs: include SentryFlutter.init documentation

[alestiago]

📜 Description

Adds documentation based on the information from #2073 and #2087.

#skip-changelog

💡 Motivation and Context

#2073 and #2087

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.50%. Comparing base (c3b8a98) to head (03ec9ea).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2093      +/-   ##
==========================================
- Coverage   95.25%   92.50%   -2.76%     
==========================================
  Files          54       58       +4     
  Lines        1791     1960     +169     
==========================================
+ Hits         1706     1813     +107     
- Misses         85      147      +62     

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[buenaflor]

Thanks! 🚀