Add documentation for the OpenWeatherMap agent (#111)

Author: dominiquekleeven

docs/user-guide/agents-protocols/openweathermap.md

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+---
+sidebar_position: 17
+---
+
+# OpenWeatherMap
+
+This agent enables integration with [OpenWeatherMap](https://openweathermap.org/), providing access to weather data via the [One Call 3.0 API](https://openweathermap.org/api/one-call-3#current).
+
+- Periodically retrieves **current**, **hourly**, and **daily** forecast data from OpenWeatherMap 
+- Updates the current and predicted values of linked attributes 
+- Supports both current conditions and forecasted hourly and daily data 
+- Can optionally provision a complete **Weather Asset** with preconfigured attributes and links 
+
+---
+
+## Agent configuration
+The following table describes the supported configuration attributes and their purpose:
+
+| Attribute | Description | Required | Default |
+| ---------- | ------------ | -------- | -------- |
+| **API key** | API key for the OpenWeatherMap API. Must have access to the One Call 3.0 API. | Yes | - |
+| **Polling millis** | Polling interval in milliseconds. | No | `3600000` |
+
+**Polling millis** is an optional attribute and does not need to be set, the agent will by default update the weather values every hour.
+
+---
+
+### Provision weather asset
+**Provision weather asset** is a helper attribute that provisions a **Weather Asset** with the appropriate attributes and agent links when checked. After the asset is created, the checkbox resets automatically. The provisioned asset will appear as a child of the agent.
+
+---
+
+## Agent Link
+For attributes linked to this agent, the following table describes the supported **Agent Link** fields, in addition to the standard [Agent Link](overview.md#agent-links) fields:
+
+| Attribute | Description | Required | Default |
+| ---------- | ------------ | -------- | -------- |
+| **Weather property** | The weather property to use for the linked attribute. | Yes | - |
+
+Example when linking an attribute that represents the current/forecasted temperature, the **Weather property** should be set to `TEMPERATURE`.
+
+---
+
+## Important notes
+- The asset containing the linked attribute(s) **must have a location set**. The agent uses this location to retrieve weather data from the OpenWeatherMap API. 
+- Weather values of the linked attributes are updated each time the polling interval is reached. It may take some time before the linked attributes are updated. Note, disabling and re-enabling the agent will trigger an immediate update of the linked attributes.