Add CASE expression documentation

[arnaud-lacurie]

Summary

Adds complete documentation for CASE expressions including both Simple and Searched forms.

Features documented

• Searched CASE - Conditional evaluation with WHEN/THEN/ELSE
• Simple CASE - Value matching against an expression
• CASE in SELECT, WHERE, and ORDER BY clauses
• Nested CASE expressions
• NULL handling and type compatibility

Key sections

• Comprehensive syntax and parameters for both forms
• Examples showing stock categorization, discounts, filtering, custom sorting
• Important notes on evaluation order, NULL handling, and performance
• Comparison of Simple vs Searched CASE with use case guidance

Files

• CASE.rst with detailed examples and best practices
• CASE.diagram for syntax visualization
• case-documentation-queries.yamsql with 6 test cases
• Updated DQL.rst and DocumentationQueriesTests.java