Use git submodules instead of committing files from the SLIP repository

[rekmarks]

Instead of committing the raw markdown we could use a git submodule. We should probably do that.

[ajeetd]

@FrederikBolding ancient ticket since perhaps the very beginning, I'm going to close it unless there's objection this week.

[ajeetd]

Closed based on above comment.

[FrederikBolding]

We still should do this.