Documentation for new OpenTherm component

[olegtarasov]

Description:

This PR adds documentation for the new OpenTherm component in ESPHome.

Pull request in esphome with YAML changes (if applicable): esphome/esphome#6645

Checklist:

• [x] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
• [x] Link added in /index.rst when creating new documents for new components or cookbook.

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[esphome[bot]]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks :+1:

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[olegtarasov]

@kbx81 Thank you for the review! I've applied your suggestions and wrapped most lines at 120 characters.