Refactor "user config templates" to be in the main zmk repo

[petejohanson]

Currently the setup.sh script (docs/static/setup.sh) uses separate git repos:

• https://github.com/zmkfirmware/zmk-config-split-template
• https://github.com/zmkfirmware/zmk-config-template

To clone the template, and then populate the initial user config repo being created by the script. This makes changing setup.sh harder, since it requires coordinate of PRs against multiple repos. It also means updates to the templates themselves are PRs against multiple repos, easier to miss some, etc.

We should refactor setup.sh to clone just a subdirectory of the main zmk repo, e.g. zmk/templates/split, zmk/templates/single, or zmk/templates/onboard, and use that to kickstart the user repo.

We'll need to leverage newer Git functionality for this, e.g.: https://stackoverflow.com/questions/600079/how-do-i-clone-a-subdirectory-only-of-a-git-repository/52269934#52269934

So we should also check the Git version before continuing.

ToDo:

• Move template files into templates/* subdirectories.
• Add git version check to setup.sh
• Once ready to create the user repo, get the initial baseline files by cloning the appropriate subdirectory of the zmk repo.

[caksoylar]

Related: #1178. I believe this would be addressed by #1183 as well.