Extend the snippet verification system to cover docs/*.md in addition to
README.md. Create runnable example files for all 18 new code snippets.

Script changes:
- Rename update_readme_snippets.py -> update_doc_snippets.py
- Process README.md + all docs/*.md files via glob
- Update CI job and pre-commit hook references

New snippet files (18):
- examples/snippets/servers/: audio, binary resources, resource templates,
  resource subscriptions, embedded resource results (text + binary),
  prompt embedded resources, prompt image content, prompt change notifications,
  tool change notifications, elicitation enum/complete, set logging level,
  JSON schema
- examples/snippets/clients/: roots, SSE client, logging, cancellation

All snippet-source tags verified: uv run scripts/update_doc_snippets.py --check