[Modbus Controller] Document new automations `on_online` and `on_offline`

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Description:

PR esphome/esphome#7417 added 2 new automations on_online and on_offline. This PR documents it.

Pull request in esphome with YAML changes (if applicable): esphome/esphome#7417

Checklist:

• [X] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• [ ] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [ ] Link added in /index.rst when creating new documents for new components or cookbook.

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