Fix auto-fixable markdownlint violations

This fixes all violations that can be fixed by using
`npm run lint:markdown -- --fix`.

This also updates the markdownlint configuration to require dashes in
unstyled lists. This results in the least amount of changes to Markdown
files. The default setting results in rewriting the complete CHANGELOG
file.

.markdownlint.json

@@ -1,4 +1,7 @@
 {
+  "ul-style": {
+    "style": "dash"
+  },
   "MD013": false,
   "MD041": false
 }