fix: Tools return rg.Stop

[monoxgas]

• Allow tools to return rg.Stop in addition to raise it
• Some other docs cleanup.

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Generated Summary

• Added explicit handling for Stop exceptions by raising them when encountered in tool call results (in docs/api/tools.mdx and rigging/tools/base.py) to ensure immediate termination.
• Updated the tools documentation to clarify that users can either raise or return a Stop exception, with a tip explaining the benefit of returning the exception to avoid interference from surrounding code.
• Revised the generators documentation to include a detailed note on generator identifiers and expanded examples for better clarity.

This summary was generated with ❤️ by rigging