I need to provide my own private key when using certbot. I understand this is already doable (sort of) by taking on the effort of just generating the CSR myself.
However, this FAQ blurb leads me to believe I don't need to, and that there's currently a way to provide just a private key:
Yes, you can obtain a certificate for an existing private key (if the key is an appropriate type and size), and, if you want, you can use an existing CSR.
Now, either there's some documentation out that that will allow me to only provide a key without generating a whole CSR, or this FAQ answer is very misleading
Most likely you’re accidentally trying to push to the HTTPS version of the repository. Do you have a GitHub account? If so, you can create a fork, push to the fork, and submit a PR from there.