Add new color temperature feature documentation

[danielkent-net]

Description:

Adds documentation for proposed color temperature features:

• Add relative_color_temp option to light.dim_relative service
• Add {get,set}_color_temperature_kelvin helper functions for lights

Pull request in esphome with YAML changes (if applicable): esphome/esphome/pull/5005 esphome/esphome/pull/5006

Checklist:

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

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[github-actions[bot]]

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.

[danielkent-net]

Still planning on finishing this up - just haven't had time to finish work on the esphome-side code yet.