chore(devcontainer): update mkdocs feature source and improve setup docs

- Update mkdocs feature from devcontainers-contrib to devcontainers-extra (ref: https://github.com/microsoft/vscode-remote-release/issues/11046)
- Clarify fork workflow in devcontainer tutorial

Signed-off-by: ElaineDeMattosSilvaB <elaine.de-mattos-silva-bezerra@deutschebahn.com>

.devcontainer/devcontainer.json

@@ -4,7 +4,7 @@
   "build": { "dockerfile": "Dockerfile" },
   "features": {
     "ghcr.io/devcontainers/features/common-utils:2.5.6": {},
-    "ghcr.io/devcontainers-contrib/features/mkdocs:2": {}
+    "ghcr.io/devcontainers-extra/features/mkdocs:2": {}
   },
   "postCreateCommand": "bash .devcontainer/setup.sh",
   "hostRequirements": {