add weathersource docs

Author: pierrepetersmeier

docs/readthedocs/models/input/additionaldata/weathersource.md

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+# WeatherSource
+
+The **`WeatherSource`** is an abstract class that provides a standardized interface for accessing time-series weather data. It serves as a foundation for concrete implementations that fetch data from various providers (e.g., CSV files or SQL databases).
+
+A `WeatherSource` relies on two key components:
+* **[`IdCoordinateSource`](/additionaldata/idcoordinatesource)**: Used to resolve a numeric **coordinate ID** from the source data into a geographic `Point` object.
+* **`TimeBasedWeatherValueFactory`**: Used to construct `WeatherValue` objects from the raw data fields.
+
+***
+
+## Information
+
+The source data for any `WeatherSource` implementation is expected to contain the following information.
+
+```{list-table}
+   :widths: auto
+   :class: wrapping
+   :header-rows: 1
+
+   * - Attribute
+     - Remarks
+
+   * - **`coordinateid`**
+     - An **integer ID** for a specific geographic coordinate. This ID is used to look up the actual `Point` via the `IdCoordinateSource`.
+
+   * - **Time**
+     - The specific timestamp of the weather data, typically as a `ZonedDateTime`.
+
+   * - **Weather Data**
+     - The meteorological values, such as solar irradiance (`direct` and `diffuse`), temperature, and wind data (`speed` and `direction`).