chore(ci): update github actions to latest version

[btkostner]

This updates the GitHub actions to the latest version, uses cache everywhere (since it was already used a couple of places), and updates the deploy step to use actions instead of deploying via a git branch. This should improve clone performance a bit by not having an orphan with all of the images stored in git.

This will require you to change the github pages settings to deploy via "Github Actions" instead of "Deploy from a branch", and set the cname to support.system76.com.

https://github.com/system76/docs/settings/pages