Add assist_sensor to sensors documentation

[ujjol1234]

Proposed change

Type of change

• Document existing features within Home Assistant Companion App
• Document new or changing features for which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Changes to the backend of this documentation
• Remove stale or deprecated documentation

Checklist

• I have read and followed the documentation guidelines.
• I have verified that my changes render correctly in the documentation.

Additional information

• Link to relevant existing code or pull request: Expose status of Assist as a sensor android#6098

[jpelgrom]

• Add a beta label to the sensor as documentation is merged when your sensor is added to the beta app (example). We'll remove it when there is a new stable app release.
• The sensor states do not match the states in your Android PR. Why not use the same description as in the app?

[home-assistant]

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

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