doc: Document the "dashboard" option in manifest.json
doc: Document the "dashboard" option in manifest.json

This was undocumented, but now that people are creating more
dashboards we should just document it.

Also document the "order" option for ordering dashboards
menu items, and tools.

Closes #5832
Reviewed-by: Peter <a316049dd34607c62d4611d63ab22a6454add223@redhat.com>

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