Simplify setup instructions in CONTRIBUTING.md (#15672)

## Summary

Replace redundant setup steps with `make` command, which handles copying
`.env.example`, running `yarn`, and setting up git configuration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: runningcode

CONTRIBUTING.md

@@ -9,8 +9,7 @@
 We use Next.js, `yarn` and `volta` to manage the environment.
 
 ```
-cp .env.example .env.development
-yarn
+make
 
 # Start dev server for user docs
 yarn dev