Document coding standards

[jensren]

What: put some coding standards guidelines on the wiki

Why: hopefully have a cleaner code base and spend less time advising people manually on what to do. Instead we can just say 'please refer to this part of the wiki'

Implementation details: some things we want to write down (more experienced devs might already know this, but not all devs will):

• How to fork and PR (link to some guide)
• PR guidelines: what should they write
• Pull from main frequently, and pull and resolve conflicts before submitting a PR
• All PRs must not have lint warnings and must build to be accepted
• Commit message guidelines: probably link to some guide, and an example of what we want in case they don't want to read the guide
• Run the linter before pushing (how to use eslint, running yarn lint)
• Other things?