added embed=true URI parameter to show stramlit in iframe, added supp…

[jonco5555]

…ort in the plugin for already exists URI parameters in the URL

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Note

Improves chatbot URL handling and updates docs/config.

• Updates src/mkdocs_chatbot/overrides/main.html to append project=<name> using ? or & based on whether chatbot_url already has query params
• Sets example chatbot plugin URL in mkdocs.yaml to https://mkdocs-chatbot.streamlit.app/?embed=true
• Adds documentation in docs/backend_setup.md explaining project name detection and URL construction
• Tweaks uv.lock Python requirement to >=3.12, <4.0

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