doc: Fix links (#6703)

CONTRIBUTING.md

@@ -2,11 +2,11 @@
 
 Thank you for your interest in building this thing together with us. We're really happy with our active community and are glad that you're a part of it. There are many ways to contribute to our project, but given the fact that you're on Github looking at the code for The Things Stack for LoRaWAN, you're probably here for one of the following reasons:
 
-- **Asking a question**: If you have questions, please use the [forum](https://www.thethingsnetwork.org/forum/). We have a special [category for The Things Stack](https://www.thethingsnetwork.org/forum/c/network-and-routing/v3).
+- **Asking a question**: If you have questions, please use the [forum](https://www.thethingsnetwork.org/forum/). We have a special [category for The Things Stack](https://www.thethingsnetwork.org/forum/c/ttn-network/v3/90).
 - **Requesting a new feature**: If you have a great idea or think some functionality is missing, we want to know! The only thing you have to do for that is to [create an issue](https://github.com/TheThingsNetwork/lorawan-stack/issues) if it doesn't exist yet. Please use the issue template and fill out all sections.
 - **Reporting an issue**: If you notice that a component of The Things Stack is not behaving as it should, there may be a bug in our systems. In this case you should [create an issue](https://github.com/TheThingsNetwork/lorawan-stack/issues) if it doesn't exist yet. Please use the issue template and fill out all sections. For sensitive (security) issues, you can [contact us directly](#security-issues).
 - **Implementing a new feature or fixing a bug**: If you see an [open issue](https://github.com/TheThingsNetwork/lorawan-stack/issues) that you would like to work on, let the other contributors know by commenting in the issue.
-- **Writing documentation**: If you see that our documentation is lacking or incorrect, it would be great if you could help us improve it. This will help users and fellow contributors understand how to better work with our stack. Better documentation helps prevent making mistakes and introducing new bugs. Our documentation is spread across a number of places. Code documentation obviously lives together with the code, and is therefore probably in this repository. User documentation for The Things Stack that is published on [thethingsstack.io](https://thethingsstack.io), is built from the [`lorawan-stack-docs` repository]https://github.com/TheThingsIndustries/lorawan-stack-docs. More general documentation can be found on [The Things Network's official documentation pages](https://www.thethingsnetwork.org/docs). The source files for that documentation can be found in [the `docs` repository](https://github.com/TheThingsNetwork/docs).
+- **Writing documentation**: If you see that our documentation is lacking or incorrect, it would be great if you could help us improve it. This will help users and fellow contributors understand how to better work with our stack. Better documentation helps prevent making mistakes and introducing new bugs. Our documentation is spread across a number of places. Code documentation obviously lives together with the code, and is therefore probably in this repository. User documentation that is published on [the official The Things Stack documentation page](https://thethingsstack.io), is built from the [`lorawan-stack-docs` repository](https://github.com/TheThingsIndustries/lorawan-stack-docs). More general documentation can be found on [The Things Network's official documentation pages](https://www.thethingsnetwork.org/docs). The source files for that documentation can be found in [the `docs` repository](https://github.com/TheThingsNetwork/docs).
 
 If you'd like to contribute by writing code, you'll find [here](DEVELOPMENT.md) how to set up your development environment.