docs: rebrand to Claude SDK and streamline README

- Rename "Anthropic Python API library" to "Claude SDK for Python"
- Replace "Anthropic API" with "Claude API" in README
- Streamline README: remove inline usage examples, centralize docs at platform.claude.com
- Update getting started example to use claude-opus-4-6-20250219
- Add documentation contribution link to CONTRIBUTING.md
- Point users to platform.claude.com/docs/en/api/sdks/python for full docs

Author: nikblanchet

CONTRIBUTING.md

@@ -1,3 +1,7 @@
+## Contributing to documentation
+
+The documentation for this SDK lives at [platform.claude.com/docs/en/api/sdks/python](https://platform.claude.com/docs/en/api/sdks/python). To suggest changes, open an issue.
+
 ## Setting up the environment
 
 ### With `uv`