Create a new source with script or GitHub action

[datenreisender]

Part of NCD-795:

Add script and GitHub Actions workflow to create a source.

The added description field is not yet used, but the idea is to already maintain it and eventually show it in the launcher.

[boundlesscalm]

While I'm a bit worried that we will forget the context this was created in and feel like just creating the source if it doesn't exist would be nicer, I do think it is better to have this as an explicitly separate action.
Makes it so that we have to do this consciously, and in the worst case it is also a fail-safe if we give it the wrong source to upload to.

[datenreisender]

@boundlesscalm I also mentioned this action and script in the documentation about releasing an app, making it more likely to remember that we have it: https://nordicsemi.atlassian.net/wiki/spaces/WYLND/pages/1363739420/nRF+Connect+for+Desktop+App+Release+Procedure#Release-to-an-unforeseen-source

And even if we forgot this is here or why and e.g. we rather create a source manually by creating a source.json in the right place, I see little harm in having this action/script.

Also, if I really do create a new UX for adding sources (NCD-1188) I probably will add new metadata on Artifactory to find all available sources, which will make this script even more valuable to use.