Add documentation for Modbus cover

[vzahradnik]

Proposed change

This PR adds documentation for the Modbus cover entity.

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase: Add Modbus cover core#33642
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue:

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[vzahradnik]

@frenck Due to recent changes in the parent PR, I reworked the documentation:
Before this change, cover configuration was part of the cover integration (which is not allowed for new integrations anymore):

cover:
  - platform: modbus
    covers:
      - name: Door1
        hub: hub1
...

After the change, the cover configuration is part of the modbus platform:

modbus:
  hubs:
    - name: hub1
      type: tcp
      host: IP_ADDRESS
      port: 502

  covers:
    - name: Door1
      hub: hub1
      ...

I'd like to ask if it's OK to create a separate page for the cover component. I checked other integrations, and they put the documentation into the main page, which is Modbus in my case. However, the cover documentation contains many examples to clarify it's use, and I think it's better to keep it in a separate page.

Thanks!

[frenck]

Awesome, thanks @vzahradnik 👍