docs(auditlog_ng): update user-guide

[jplbrun]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Reworks the auditlog_ng user guide (src/sap_cloud_sdk/core/auditlog_ng/user-guide.md) so it follows the conventions used by the other module user guides, and corrects several statements that did not match the actual client behavior.

Structure aligned to the house convention

• Retitled to # Audit Log NG User Guide and merged the redundant Overview into the single intro paragraph.
• Renamed Prerequisites → Installation, added a Quick Start section (minimal create_client(tenant=...) → build event → send() → close()), and grouped the walkthrough, full example, one-off sends, serialization table, and auto-injection under a single Usage section.
• Normalized headings to Title Case, removed the ✅/❌ emoji from the config tables in favor of plain Yes/No, and dropped the duplicate import block and the module-specific Running the Unit Tests section.

Corrected content (verified against the code)

• Validation section replaced with an Error Handling section that lists the exceptions send() actually raises: ValidationError on schema failure (subclass of AuditLogNGError), ValueError for an invalid common.tenant_id, and RuntimeError on a closed client. The previous text incorrectly claimed a tenant_id UUID check and used the wrong exception types.
• Fixed the config-path description to reflect the real precedence (config= → tenant → explicit kwargs).
• Matched the validation regex shown in the docs ([a-zA-Z0-9._/~-]) to the pattern compiled in config.py.

Recommended setup updated

• Automatic tenant/user injection now documents bootstrap(app) as the recommended way to wire IAS context + telemetry for new apps, with StarletteIASTelemetryMiddleware retained under a Legacy subsection for apps not yet on bootstrap().

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. Open src/sap_cloud_sdk/core/auditlog_ng/user-guide.md and read it top to bottom

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

N/A

Additional Notes

None