(Very) minor improvements to CONTRIBUTING.md

[Kostas-F]

Summary

Some minor problems I spotted with the md file that would be quick to fix . All trivial, but sorted in my perceived order of "importance"

https://github.com/TheThingsNetwork/lorawan-stack/blob/v3.28/CONTRIBUTING.md:

1. Line 5: The link to the forum category is dead. I believe the correct url now is https://www.thethingsnetwork.org/forum/c/ttn-network/v3/90

2. Line 9: The site referenced as the location of user documentation is thethingstack.io. It properly redirects to the thethingindustries docs location so no problems there. However correcting the user facing link would help with dissambiguiation.

3. Also Line 9: Missing parenthesis in the docs repository link.

Why do we need this ?

To assist users in finding the reference locations with the least ammount of friction

What is already there? What do you see now? What would you like to see?

The refenced minor issues and their corrections respectively.

[Kostas-F]

This is in reference to the CONTRIBUTING.md in The Things Network lorawan-stack repository, I just realized the issue opened in this repository (Things Industries) when I clicked the documentation request in the issue dialog. I hope it is not in the wrong place

[nejraselimovic]

Hey @Kostas-F, thanks for reporting, this should've been submitted to TTN and TTI lorawan-stack repos indeed, but never mind, I've submitted PRs to fix this on both places:

https://github.com/TheThingsIndustries/lorawan-stack/pull/3951
TheThingsNetwork/lorawan-stack#6703

[Kostas-F]

Hi @nejraselimovic I apologize for the confusion, thank you for taking the time.