Integrate Vale.sh into the documentation

[settermjd]

Vale is an open source prose linter and validator. From what I can tell, it seems to be one of the best and most used tools for checking documentation.

This change adds the core configuration, along with a custom library for the Slim project. I don't have access to the repo's settings, so am not able to provide anything in the way of adding it to the build/deployment process. Happy to talk this through, however.

[l0gicgate]

Will not be merging this. We don't need this. Thank you though.

[akrabat]

Reopened - communication failure on our side.