You might as well just write instructions in English in any old format, as long as it's comprehensible. Exactly as you'd do for human readers! Nothing has really changed about what constitutes good documentation. (Edit to add: my parochialism is showing there, it doesn't have to be English)
Is any of this standardization really needed? Who does it benefit, except the people who enjoy writing specs and establishing standards like this? If it really is a productivity win, it ought to be possible to run a comparison study and prove it. Even then, it might not be worthwhile in the longer run.
A skill is essentially just a markdown file, containing whatever instructions you want, possibly linking to other markdown files and/or scripts to avoid context pollution.
What skills give you is autodiscovery. You need to somehow tell the agent that documentation exists and when it should be looked at, and that's exactly what the skills standard does. It's a standardized format for documentation that harnesses can automatically detect and inform agents about, without them having to do many useless calls on every single turn to see if there are any skills present.