add reactive power to cse7766

[nkinnan]

Description:

Documents a new sensor under the CSE7766 component.

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#7257

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.