dimmer: Add OXT 1-channel dimmer component type

[michau-krakow]

Description:

Related issue (if applicable):

Pull request in esphome with YAML changes (if applicable): esphome/esphome#6021

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.

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