docs: update CLI reference and troubleshooting for board and actionable errors

[aheritier]

Summary

Catches two documentation gaps from PRs merged in the last 36 hours.

Changes

• docker agent board CLI reference (docs/features/cli/index.md): PR feat: add docker agent board, a Kanban TUI for orchestrating agents #3451 shipped a full feature page for the new Kanban board but omitted the command from the master CLI reference. This commit adds the docker agent board section with its usage, requirements, and a link to the feature page.

• Actionable model/credential errors (docs/community/troubleshooting/index.md): PR feat(cli): actionable errors for missing models and credentials (phase 1 of #3442) #3452 introduced clear, actionable error messages when model credentials are missing or Docker Model Runner is unreachable. No documentation was written at the time. This commit adds a troubleshooting section covering those three scenarios with example error text and a docker agent doctor diagnosis tip.

Source PRs

• feat: add docker agent board, a Kanban TUI for orchestrating agents #3451
• feat(cli): actionable errors for missing models and credentials (phase 1 of #3442) #3452

Reviewer notes

Two low-severity/low-confidence style findings were raised and deliberately left as-is:

• Placement of docker agent board: reviewer suggested grouping it with other top-level commands (before serve). Placed after serve chat per the explicit task spec.
• Windows caveat: docker agent board doesn't work on Windows (no tmux). The feature page at docs/features/board/index.md notes only "Requires tmux and git"; the CLI reference mirrors that same level of detail for consistency.