Add entry for inventree-stock-forecasting plugin (#242)

* Add entry for inventree-stock-forecasting plugin

* Remove expired link

_repo/inventree-stock-forecasting.md

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+---
+name: inventree-stock-forecasting
+author: SchrodingersGat
+license: MIT
+open_source: true
+stable: true
+maintained: true
+pypi: true
+package_name: inventree-stock-forecasting
+github: https://github.com/inventree/inventree-stock-forecasting
+issue_tracked: https://github.com/inventree/inventree-stock-forecasting/issues
+website: https://inventree.org
+categories: Reporting
+tags: stock forecasting data
+---
+
+Stock Forecasting for InvenTree
+
+A plugin which generates stock forecasting data for parts in InvenTree
+
+## Installation
+
+The easiest way to install the plugin is via the InvenTree user interface.
+
+Alternatively, you can install the plugin manually as follows:
+
+```
+pip install inventree-stock-forecasting
+```
+
+## Documentation
+
+Refer to the [plugin documentation](https://github.com/inventree/inventree-stock-forecasting) for further instructions and information.