docs(readme): add missing modules to key features and usage guides

[thiagob]

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

Add Agent Gateway, Agent Memory, Audit Log NG, Extensibility, and IAS to the Key Features list
Add usage guide links for all 5 missing modules plus the previously unlinked AI Core module to the Usage Guides section

Related Issue

Closes #<issue_number>

(Link to the GitHub issue this PR addresses)

Type of Change

Please check the relevant option:

• 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

Describe how reviewers can test your changes:

1. All links in the Usage Guides section resolve to existing user-guide.md files
2. No module with a user-guide.md under src/sap_cloud_sdk/ is missing from the README

Checklist

Before submitting your PR, please review and check the following:

• 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

If this PR introduces breaking changes, please describe:

• What breaks
• Migration path for users
• Alternative approaches considered

Additional Notes

Add any additional context, screenshots, or information that would help reviewers.