docs: add user documentation for core organizer concepts

Add three documentation pages:
- Leden vs Personen: explains the difference between platform members and event participants
- Crowd Types: full reference for the 7 person types with access matrix
- Diensten plannen: enhanced with 4-layer model, assignment strategies, and overbezetting

Also adds placeholder images to fix VitePress build and updates sidebar navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/.vitepress/config.ts

@@ -30,6 +30,7 @@ export default defineConfig({
           items: [
             { text: "Introductie", link: "/guide/getting-started" },
             { text: "Kernconcepten", link: "/guide/concepts" },
+            { text: "Leden vs Personen", link: "/guide/leden-vs-personen" },
             { text: "Woordenlijst", link: "/guide/glossary" },
           ],
         },