Add SiLA 2 device discovery via mDNS

[rickwierenga]

Summary

• Add pylabrobot.sila package with mDNS-based discovery of SiLA 2 instruments on the local network
• discover() listens for _sila._tcp.local. services and returns SiLADevice objects with host, port, and name
• Includes a CLI tool (python -m pylabrobot.sila.discovery) for terminal-based discovery
• Add API docs and a user guide notebook

Test plan

• Verify discover() finds SiLA 2 devices on a network with active instruments
• Verify CLI tool outputs tab-delimited results
• Verify docs build cleanly

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