Add learning journey contribution guidelines

[zmitchell]

This PR adds materials for guiding contributors towards the workflow we'd like them to follow when submitting outlines and drafts of the tutorials in the learning journey tutorial series.

[zmitchell]

The check that's failing here is complaining about the tutorial template not being included in the toctree. I'm wondering if we can somehow whitelist that since I think this is where the template belongs. Otherwise I can move the template to a different directory.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-06-15-learning-journey-working-group-meeting-notes-13/29333/1

[fricklerhandwerk]

Looks good, also you know best given it's for internal use. Needs a rebase.

[zmitchell]

Apparently it's been a while since I pulled upstream so I see that the "Guidelines" section now lives in a "Writing Style" document. I think it makes sense to merge that document with style-guide.md, so I'll do that and request another review.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-07-06-learning-journey-working-group-meeting-notes-16/30210/1

[zmitchell]

This build is currently failing because of a 502 bad gateway on bob.build in influences.md. It's not clear to me whether that link is temporarily broken or just dead. Either way I'm not sure it should block this PR.