Add aggregate and scalar function documentation

[arnaud-lacurie]

Summary

Documents standard SQL aggregate and scalar functions with comprehensive examples and runnable tests.

Aggregate Functions

Added documentation for:

• COUNT - Count rows or non-NULL values
• SUM - Sum of non-NULL values
• AVG - Average (returns DOUBLE)
• MIN - Minimum value
• MAX - Maximum value

Scalar Functions

Added documentation for:

• GREATEST - Returns maximum value from expressions
• LEAST - Returns minimum value from expressions
• COALESCE - Returns first non-NULL value

Documentation Structure

• Created .rst documentation files with syntax diagrams, parameters, returns, and examples
• Created .diagram files for railroad syntax diagrams
• Added test coverage in DocumentationQueriesTests.java
• Created runnable .yamsql test files with 21 aggregate function tests and 6 scalar function tests

Testing

All documentation examples are tested and verified to work correctly.