docs: add dedicated Burr UI server documentation (issue #272)

[enu-emu]

Summary

Closes #272

• Creates docs/concepts/ui.rst with a full guide to the Burr UI server: installation, CLI options (with examples), demo data walkthrough, step-by-step descriptions of every UI feature, notebook/Colab launch, FastAPI embedding, and how to wire up tracking so your app appears in the UI
• Registers the new page in docs/concepts/index.rst under the Concepts toctree, positioned after tracking

What's in the new page

• Installation — pip install "apache-burr[start]" with a note on the Python 3.13 / pandas compatibility fix
• Demo Data and First Launch — explains the four auto-copied demo projects so new users can explore immediately
• Command-Line Options — list-table covering --port, --host, --no-open, --no-copy-demo_data, --backend, and --dev-mode with usage examples
• What You Can Do — seven subsections (Browse Projects, Explore Demos, Inspect Steps, Compare State, View Source, Replay a Run, Watch Live) each with a "Try it:" instruction using the built-in demo data
• Launch from a Notebook — %burr_ui magic with Google Colab instructions
• Embed in FastAPI — mount_burr_ui example
• Making Your App Appear — .with_tracker() setup with a cross-reference to tracking

Testing

• make html in docs/ — Sphinx build passes, zero new warnings from the new files
• New page appears in the Concepts navigation in the built HTML output
• All CLI flags verified against burr --help
• All UI features verified manually using the bundled demo projects