Document the modbus server feature

[JeroenVanOort]

Description:

Related issue (if applicable): fixes esphome/feature-requests#708

Pull request in esphome with YAML changes (if applicable): esphome/esphome#4874

Checklist:

• 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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[github-actions]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[nagyrobi]

I fixed the conflicts, but still, there's a contradiction:

modbus_controller.rst

To choose the role, set the role attribute of the Modbus Component upon which this modbus_controller component relies. client is the default.

modbus.rst

role (Optional, string): The role of this component, client or server. Defaults to server.

I'll let the author to clear this...

[esphome]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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Conflict fixed to match code PR