[native_doc_dartifier] Utility to extract code snippets and context from documentation URLs

[marshelino-maged]

Utility that parses a given documentation URL (e.g., Android docs) and extracts:

• Code snippets embedded in the page.

• Contextual information, such as surrounding text.

This is useful because LLM-generated code is based on a snippet from the docs, but this snippet may fail to include required context (e.g., missing variable initialization).