ci(ecosystem): switch vitepress to fengmk2 fork + fix-playwright-chromium branch

Pointing at fengmk2/vitepress so the playwright-chromium upgrade can land on
feat/vite-8-and-fix-playwright-chromium without waiting on the upstream
vuejs/vitepress merge.

Author: fengmk2

ecosystem-ci/repo.json

@@ -38,8 +38,8 @@
     "hash": "0d3912b73d3aa1dc8f64619c82b3dacb0769e49e"
   },
   "vitepress": {
-    "repository": "https://github.com/vuejs/vitepress.git",
-    "branch": "feat/vite-8",
+    "repository": "https://github.com/fengmk2/vitepress.git",
+    "branch": "feat/vite-8-and-fix-playwright-chromium",
     "hash": "9330f228861623c71f2598271d4e79c9c53f2c08",
     "playwright": true
   },