chore: Mention developer docs in Contributing (#12101)

Explain how to start the dev server for the developer docs in the
Contributing.md and not just the environment.mdx.

Author: philipphofmann

CONTRIBUTING.md

@@ -11,7 +11,12 @@ We use Next.js, `yarn` and `volta` to manage the environment.
 ```
 cp .env.example .env.development
 yarn
+
+# Start dev server for user docs
 yarn dev
+
+# Start dev server for developer docs
+yarn dev:developer-docs
 ```
 
 With that, the repo is fully set up and you are ready to open local docs under http://localhost:3000