deps: Switch docz-plugin-css implementation

The current version of docz-plugin-css uses node-sass v4, which is
incompatible to node v16+.

Given that pi-ui does not use any sass files, this commit switches the
docz-plugin-css dependency to a forked version which drops node-sass and
is thus compatible to node v16+.

package.json

@@ -51,7 +51,7 @@
     "cross-env": "^7.0.3",
     "docz": "^1.2.0",
     "docz-core": "^1.2.0",
-    "docz-plugin-css": "^0.11.0",
+    "docz-plugin-css": "https://github.com/matheusd/docz-plugin-css",
     "docz-theme-default": "^1.2.0",
     "eslint": "^7.5.0",
     "eslint-config-prettier": "^8.3.0",