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:

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

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[kbx81]

Mostly clean-up & copy fixes.

Also, please wrap all lines at no more than 120 characters.

[esphome]

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

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[kbx81]

Couple other things I missed earlier -- for pins in documentation, we use GPIOXX to prompt the user to specify their own pins.

[FreeBear-nc]

You describe values such as t_set as inputs - Surely they are outputs.

index 819120c9..91945ae3 100644
--- a/components/opentherm.rst
+++ b/components/opentherm.rst
@@ -147,7 +147,7 @@ output and number components by default:
 - ``auto_min_value`` (boolean): Automatically configure the minimum value to a value reported by the boiler. Not
   available for all inputs.
 
-The following inputs are available:
+The following outputs are available:
 
 - ``t_set``: Control setpoint: temperature setpoint for the boiler's supply water (°C)
 

[olegtarasov]

@FreeBear-nc You are correct, this terminology looks confusing. It's part of the original documentation from Arthur Rump that I didn't change. But I wouldn't call them inputs either because then we can declare an input as an output, and it's confusing :) I would rewrite this whole section as following:

Numerical values
*****************

There are three ways to set a numerical value:

- As an input sensor, defined in the hub configuration:

[skipped] 

  This can be useful if you have an external thermostat-like device that provides the setpoint as a sensor.

- As a number:

[skipped] 

  This is useful if you want full control over your boiler and want to manually set all values.

- As an output:

[ skipped ]

The following numerical values are available:```