docs: document coroutine function support in invoke

[fgmacedo]

Summary

• Updates the invoke execution model section to describe async-aware handler routing (coroutines awaited directly, sync handlers via executor)
• Adds a new "Coroutine functions" section with doctests for both raw async def and IInvoke with async def run()
• Adds 3.1.0 release notes entry for the feature (feat: support coroutine functions as invoke targets #611, fixes Does Invoke always run in a thread? #610)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f54d9f5) to head (1023ac7).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop      #612   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         5027      5027           
  Branches       812       812           
=========================================
  Hits          5027      5027           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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