Enforce Prettier formatting in docs/config.json config files

[ptgott]

When docs contributors edit the docs/config.json configuration file within a version-specific gravitational/teleport submodule, differences in editor configuration often cause minor formatting changes that lead to larger-than-intended diffs and merge conflicts for backports.

Since we already enforce the use of prettier for JS files, we can edit our docs linting workflow to ensure that any docs/config.json file passes prettier --check. We'll also want to include some clear language in the error message instructing the author to install Prettier and run prettier -w config.json.