Fix docs links to point to stable (not latest) (#228)

extend/api.md

@@ -9,4 +9,4 @@ InvenTree ships with an extensive REST API that is used by several native applic
 - InvenTree App (Android/iOS)
 - python module
 
-The API is self-documenting. More information is provided in the [documentation](https://docs.inventree.org/en/latest/api/api/).
+The API is self-documenting. More information is provided in the [documentation](https://docs.inventree.org/en/stable/api/api/).

extend/app.md

@@ -37,7 +37,7 @@ We provide direct downloads for Android users - view our [download page via pola
 
 ### Documentation
 
-Mobile app documentation can be found on the [InvenTree documentation site](https://docs.inventree.org/en/latest/app/app/).
+Mobile app documentation can be found on the [InvenTree documentation site](https://docs.inventree.org/en/stable/app/app/).
 
 ### Code
 

extend/index.md

@@ -8,4 +8,4 @@ InvenTree is designed to be part of a larger system of tools to support both hob
 - [API](api)
 - [Integrate](integrate) into other tools
 - PK2InvenTree for [migrating from PartKeepr to InvenTree](https://github.com/rgilham/PK2InvenTree)
-- Python [package](https://docs.inventree.org/en/latest/api/python/python/) 
+- Python [package](https://docs.inventree.org/en/stable/api/python/python/) 

extend/plugin/index.md

@@ -5,7 +5,7 @@ We supply core-dev and community built plugins to get you started with extending
 
 We expose some internal Python APIs to speed up your development and keep the instance running while you discover the possibilities.
 
-You can read more about the plugin architecture, including information on developing new plugins, on our [documentation](https://docs.inventree.org/en/latest/extend/plugins/)
+You can read more about the plugin architecture, including information on developing new plugins, on our [documentation](https://docs.inventree.org/en/stable/extend/plugins/)
 
 ### Highlights