docs: Add comprehensive Javadoc comments and update README documentation

[staging-devin-ai-integration]

docs: Add comprehensive Javadoc comments and update README documentation

Summary

This PR adds comprehensive documentation to the BankingMicroservices repository:

Javadoc Comments: Added class-level, method-level, and field-level Javadoc documentation to all Java files across all 7 microservices (API-Gateway, Service-Registry, Sequence-Generator, Account-Service, User-Service, Fund-Transfer, Transaction-Service). Documentation includes @author, @version, @param, @return, and @see tags where appropriate.

README.md: Completely rewrote the README with comprehensive project documentation including architecture overview, microservices table with ports, project structure, installation instructions, configuration examples, startup order, API endpoint tables, curl examples, and authentication flow documentation.

Review & Testing Checklist for Human

• Verify ResourceNotFound exception in Transaction-Service (Transaction-Service/src/main/java/org/training/transactions/exception/ResourceNotFound.java): The Javadoc parameter order (@param message, @param errorCode) may not match the actual constructor behavior - the super call appears to swap the parameters
• Spot-check 2-3 Javadoc descriptions against actual code behavior to ensure accuracy
• Verify the API examples in README.md match the actual API request/response formats
• Confirm the port numbers listed in the README microservices table are correct

Recommended test plan: This is a documentation-only PR with no functional changes. Review can focus on accuracy of descriptions rather than runtime testing.

Notes

• All changes are documentation-only (Javadoc comments and README.md)
• No functional code was modified
• The project structure diagram in README uses ASCII art compatible with markdown rendering

Link to Devin run: https://samconfigtest.staging.devinenterprise.com/sessions/ca0e17311d8643ab86eadfb4929d1a07
Requested by: @samfert-codeium

[staging-devin-ai-integration]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
1.6% Duplication on New Code

See analysis details on SonarQube Cloud