For Docusaurus, it needs a meta data named `sidebar_position`, so we copy all `weight` under `menu/sidebar` as top meta key.
Add `markdownlint` linter and fix issues. Config is based on the one from electron's repo with a few rules relaxed.
Co-authored-by: 6543 <6543@obermui.de> Co-authored-by: Lauris BH <lauris@nix.lv>
