Function validate_content 

pub fn validate_content(content: &Content) -> Vec<ValidationError>

Validate content structure and rules.

This validates:

• Block structure (correct children types)
• Unique block IDs
• Required fields
• Heading levels (1-6)
• List items only in lists
• Table rows only in tables
• Table cells only in rows

Errors

Returns a vector of validation errors if any are found.