docs: Add publishing guide

[joaquimrocha]

Here are the docs for the plugin publishing guide.
After putting these together, I have 2 remarks:

1. I still feel like the guide would make more sense if we add the repo in ArtifactHub and only then add the files to the github page, same with the package. Just would feel more natural to me: I create a repo in AH, then enable it.
2. The guide is for publishing on ArtifactHub which has little to do with Headlamp and more to do with the Plugin Catalog, but it's important enough that I think it's okay to have it here. I added some text explaining that it's not about discoverability on Headlamp core but rather in the plugin.

cc/ @yolossn

[yolossn]

Just would feel more natural to me: I create a repo in AH, then enable it.

I see it differently because the 'Add repository' button in ArtifactHub suggests linking an existing GitHub repository that already contains the Headlamp plugin templates, rather than creating a new repository in ArtifactHub itself. If you feel its better the other way, feel free to change it

[skoeva]

Added some suggestions for clarity