ci: disable prettier on markdown files

Created by: jhchabran

Prettier running on markdown files is not very good in terms of cost/benefit and lead to failing builds: https://buildkite.com/sourcegraph/sourcegraph/builds/109763

It trims longs lines, indent code in snippets. The former has no incidence on the output and the latter is something that should be done on your own anyway. Because it gets in the way of builds, I really don't feel that's worse it.