docs: restructure documentation following Diataxis framework

[jimsynz]

Summary

Reorganises documentation following the Diataxis framework with four distinct documentation types: Tutorials, How-to Guides, Explanation, and Reference.

Changes

New How-to Guides (documentation/how-to/)

Task-oriented guides for common operations:

• integrate-servo-driver.md - Creating hardware driver packages
• add-custom-command.md - Extending the command system
• implement-safety-callbacks.md - Safe hardware shutdown
• deploy-to-nerves.md - Embedded deployment with Igniter
• use-urdf-with-ros.md - ROS integration
• write-custom-sensor.md - Creating sensor modules
• troubleshoot-pubsub.md - Debugging message flow

Expanded Explanation (documentation/topics/)

• Rename safety.md → understanding-safety.md with explanation focus
• Add supervision-architecture.md - Why topology mirrors supervision
• Add command-system.md - Goal → Feedback → Result pattern
• Add pubsub-system.md - Hierarchical messaging

New Reference (documentation/reference/)

• message-types.md - All BB.Message payload types
• error-types.md - All BB.Error types by class
• telemetry-events.md - Observability events

Fixes

• Update mix.exs with groups_for_extras for proper ExDoc grouping
• Update README.md with organised documentation links
• Fix cross-references after safety.md rename
• Fix incorrect DSL examples throughout:
  • states section (was incorrectly shown as custom_states in settings)
  • category entities (was incorrectly shown as categories list)
  • controllers do/sensors do wrappers (were missing in several examples)
• Document that errors should use exception/1 not struct literals (for Splode backtrace capture)
• Clarify BB.Message timestamp is monotonic time, not wall-clock
• Hide internal test sensor module from documentation

Related

• Companion PRs for bb_liveview and bb_kino documentation to follow