Add linting for markdown files (#3646)

I also reworked the Linters section in `CONTRIBUTING.md` a bit and
switched the `prettier` config to a flat config.

Co-authored-by: Veeck <github@veeck.de>

.markdownlint.json

@@ -0,0 +1,6 @@
+{
+	"line_length": false,
+	"no-duplicate-heading": false,
+	"no-inline-html": false,
+	"no-trailing-punctuation": false
+}