Allow YAML replacements for embedded documentation code examples

[virtuald]

Add doc_code_blocks to replace C++ examples with Python without duplicating surrounding documentation:

 classes:
   Timer:
     methods:
       CreateStarted:
         doc_code_blocks:
           - |
             timer = Timer()
             timer.start()

• Replace blocks in source order with strict count validation.
• Support null to preserve a block and [] to disable inherited replacements.
• Support classes, methods, overloads, functions, properties, enums, and enum values.
• Preserve parameter renaming and doc_append; reject conflicting full-doc overrides.
• Require sphinxify ≥ 0.13.