docs: add architecture diagram to README

[RaymundoZa]

This PR adds an architecture diagram to the README to help new contributors and users understand how a Java application interacts with the Dapr runtime via the Java SDK.

• Shows how the app connects to the Dapr sidecar (HTTP or gRPC).
• Illustrates the APIs supported (state management, pub/sub, service invocation, bindings, actors, workflows).
• Clarifies request routing and response flow.

This addresses #915.

[cicoyle]

Would it be worth just saying Dapr API calls instead of listing them all out? I mention this because I notice you are missing some, and as we add more in the future we will become outdated if we list them all individually. My recommendation would be to note generally this section is the Dapr API calls and link to the available APIs from the Dapr Docs here: https://docs.dapr.io/reference/api/

[cicoyle]

Thanks for the PR 🎉 I think this is great! Just one comment from me

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.91%. Comparing base (d759c53) to head (7b34d0d).
⚠️ Report is 217 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1549      +/-   ##
============================================
+ Coverage     76.91%   78.91%   +1.99%     
- Complexity     1592     1936     +344     
============================================
  Files           145      237      +92     
  Lines          4843     5956    +1113     
  Branches        562      611      +49     
============================================
+ Hits           3725     4700     +975     
- Misses          821      936     +115     
- Partials        297      320      +23     

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