The previous commands did not work (these commands did not exist), so I have updated the contributing guide so that others won't face the same problem in the future.
Everyone is very welcome to contribute on the codebase of Remix. Please reach us in [Gitter](https://gitter.im/ethereum/remix)!
The easiest way to work on `remix-debugger` or `remix-solidity` is to pull the `remix-ide` (Remix IDE) repository (https://github.com/ethereum/remix-ide) which has a strong Remix integration. Then, in `remix-ide`, execute `npm install`, `npm pullremix`, `npm linkremix`.
The easiest way to work on `remix-debugger` or `remix-solidity` is to pull the `remix-ide` (Remix IDE) repository (https://github.com/ethereum/remix-ide) which has a strong Remix integration. Then, in `remix-ide`, execute `npm install`, `npm run pullremix`, `npm run linkremixsolidity`.
then `npm run build && npm run serve` to start a new Remix IDE instance (you can browse `127.0.0.1:8080`).