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>

prettier.config.mjs

@@ -0,0 +1,13 @@
+const config = {
+	overrides: [
+		{
+			files: "*.md",
+			options: {
+				parser: "markdown"
+			}
+		}
+	],
+	trailingComma: "none"
+};
+
+export default config;