Add a 'Why not Context7 or generic docs retrieval?' section

[ayhammouda]

Why

The obvious objection is: 'Why not just use Context7 / a browser / generic fetcher?' If the README does not answer that directly, users will assume this project is redundant.

Points to cover

• This is Python stdlib-focused, not broad library docs.
• Official CPython docs are the source of truth.
• Version-aware lookup matters for Python 3.10-3.14.
• Section-level retrieval saves tokens versus full-page dumps.
• Local read-only index means no API keys and no hosted dependency at query time.

Acceptance criteria

• README has a concise comparison section.
• It is honest about when users should prefer generic retrieval instead.
• No dunking on other tools; position this as specialized, not universal.