docs: add Node.js requirement, footnotes, and Obsidian plugin support table

- Add Node.js 22 prerequisite callout to installation guide
- Document footnote support via GFM plugin
- Add Obsidian community plugin support table (Dataview, Excalidraw, Leaflet)

docs/getting-started/installation.md

@@ -15,6 +15,9 @@ cd quartz
 
 ## 2. Install Dependencies
 
+> [!important]
+> Quartz requires **Node.js 22** or later. Check your version with `node -v` and upgrade at [nodejs.org](https://nodejs.org/) if needed.
+
 ```bash
 npm i
 ```