Add template Weather provider (#16681)

* initial commit

* Update source/_integrations/weather.template.markdown

Co-authored-by: Franck Nijhof <frenck@frenck.dev>

source/_integrations/weather.template.markdown

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+---
+title: "Template Weather Provider"
+description: "Instructions on how to integrate Template Weather provider into Home Assistant."
+ha_category: 
+  - Weather
+ha_release: 2021.3
+ha_iot_class: "Local Push"
+ha_qa_scale: internal
+ha_domain: template
+---
+
+The `template` integrations creates weather provider that combines integrations and an existing weather provider into a fused weather provider.
+
+There are several powerful ways to use this integration, including localizing your weather provider information with local information from temperature, humidity, pressure sensors that you own.
+
+Another use case could be using temperature and humidity from one weather plaform, with forecasts from a different one.
+
+
+## Configuration
+
+To enable a Template Weather provider in your installation, add the following to your `configuration.yaml` file:
+
+{% raw %}
+
+```yaml
+# Example configuration.yaml entry
+weather:
+  - platform: template
+    name: "my very own weather station"
+    condition_template: "sunny"
+    temperature_template: "{{ states('sensor.temperature') | float}}"
+    humidity_template: "{{ states('sensor.humidity')| float }}"
+    forecast_template: "{{ states.weather.my_region.attributes.forecast }}"
+```
+
+{% endraw %}
+
+{% configuration %}
+name:
+  description: Name to use in the frontend.
+  required: true
+  type: string
+condition_template:
+  description: The current weather condition.
+  required: true
+  type: template
+temperature_template:
+  description: The current temperature.
+  required: true
+  type: template
+humidity_template:
+  description: The current humidity.
+  required: true
+  type: template
+pressure_template:
+  description: The current air pressure.
+  required: false
+  type: template
+wind_speed_template:
+  description: The current wind speed.
+  required: false
+  type: template
+forecast_template:
+  description: Daily forecast data.
+  required: false
+  type: template
+{% endconfiguration %}
+
+## Considerations
+
+If you are using the state of a integration that takes extra time to load, the Template Weather may get an `unknown` state during startup. This results in error messages in your log file until that integration has completed loading. If you use `is_state()` function in your template, you can avoid this situation.