Add RAP documentation and reorganize data access layer docs

[oblomov-dev]

Summary

This PR adds comprehensive documentation about RAP (RESTful Application Programming Model) and its relationship with abap2UI5, while reorganizing the data access layer documentation structure for better clarity.

Key Changes

• New RAP documentation (docs/development/specific/rap.md): Explains that RAP is a full-stack programming model while abap2UI5 is a UI rendering library, clarifies the distinction between the two, documents supported access layers (ABAP SQL, CDS Views, EML, Function modules, etc.), and explains how RAP functionality can be called from outside the RAP framework with full control over commits and error handling
• Reorganized sidebar navigation: Consolidated "RAP, EML" and "CDS, SQL" sections into a single "EML, CDS, SQL" section with RAP as the primary link, improving information architecture
• Updated section labels: Changed "XLSX" to "Spreadsheet" for better clarity in the documentation sidebar

Notable Details

• The RAP documentation emphasizes that abap2UI5 runs outside the RAP framework, meaning RAP restrictions (like no explicit COMMIT WORK) do not apply, giving developers more control
• Includes practical code examples showing EML usage patterns within abap2UI5 controllers
• Clarifies that abap2UI5 is access-layer agnostic and works with any valid ABAP statement

https://claude.ai/code/session_0133KCCdNqKq8PpZRFR3jvfX