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]

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

[RaymundoZa]

✅ Updated the diagram based on the review feedback:

• Replaced the detailed list of APIs with a generic “Dapr API calls” to avoid the diagram becoming outdated as new APIs are added.
• Grouped the Sidecar and Components under “Dapr Runtime” for clarity, inspired by issue [Proposal] Migrate waitForSidecar to v1.0/healthz/outbound to provide parity with dotnet SDK #897 (but not copied).
• Added a reference link to the Dapr API documentation for the full list of available APIs.
• This keeps the diagram simple, accurate, and easier to maintain going forward.

flowchart LR
    A[End User Application] -->|HTTP or gRPC| B[Dapr Java SDK]
    B -->|Dapr API calls| C[Dapr Sidecar]
    C -->|Components| D[State Stores, Pub/Sub, Services, etc.]

    %% Optional grouping for clarity
    subgraph R[Dapr Runtime]
        C
        D
    end

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[artur-ciocanu]

@RaymundoZa thanks a lot for your contribution!

Could you please sign your commits. This a requirement for this repo. Otherwise we won't be able to merge your changes.

Thank you!

[artur-ciocanu]

@cicoyle could you please check if you are OK with the changes and approve. Thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.49%. Comparing base (d759c53) to head (3cd15ea).
⚠️ Report is 228 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1549      +/-   ##
============================================
+ Coverage     76.91%   78.49%   +1.58%     
- Complexity     1592     1913     +321     
============================================
  Files           145      215      +70     
  Lines          4843     5827     +984     
  Branches        562      649      +87     
============================================
+ Hits           3725     4574     +849     
- Misses          821      916      +95     
- Partials        297      337      +40     

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