KNOX-3318 - Add Knox LDAP Server documentation to User Guide

What changes were proposed in this pull request?

This pull request adds comprehensive documentation for the Knox LDAP Server feature (introduced in KNOX-3247 and improved in KNOX-3277) to the official Knox User Guide.

Changes include:

• Created knox-site/docs/service_ldap_server.md which provides:
  • An architectural overview of the embedded ApacheDS-based LDAP service.
  • Detailed configuration instructions for both file and proxy backends.
  • Specific guidance for Active Directory (AD) integration, including memberOf and sAMAccountName support.
  • Troubleshooting tips regarding logs and lock files.
• Updated knox-site/mkdocs.yml to include the "LDAP Server" page in the Client/User Guide navigation.
• Updated knox-site/docs/book_service-details.md to include the LDAP Server in the master list of supported services.

How was this patch tested?

• Manually verified the Markdown rendering and formatting of the new documentation.
• Validated all internal and external (JIRA) links.
• Cross-referenced all configuration property names (e.g., gateway.ldap.backend.proxy.*) with the implementation in GatewayConfigImpl.java and LdapProxyBackend.java to ensure accuracy.

Integration Tests

N/A - This is a documentation-only change. No functional code was modified.

UI changes

N/A