Add docs contributing instructions (#1779)

CONTRIBUTING.md

@@ -4,6 +4,8 @@
 
 If you would like to hack on MUI Toolpad or want to run the latest version, you can follow these steps:
 
+_If you're looking into contributing to the docs, follow the [instructions](#building-and-running-the-documentation) down below_
+
 ### Prerequisites
 
 - git
@@ -92,6 +94,28 @@ If you would like to hack on MUI Toolpad or want to run the latest version, you
   yarn prisma db push --accept-data-loss
   ```
 
+## Building and running the documentation
+
+1. If you haven't already, install the project dependencies using
+
+   ```sh
+   yarn
+   ```
+
+1. To start the documentation application in dev mode run
+
+   ```sh
+   yarn docs:dev
+   ```
+
+   If all goes well it should print
+
+   ```sh
+   ready - started server on 0.0.0.0:3003, url: http://localhost:3003
+   ```
+
+1. Open the docs application in the browser [http://localhost:3003/toolpad](http://localhost:3003/toolpad)
+
 ## Sending a pull request
 
 Please have a look at our general guidelines for sending pull requests [here](https://mui-org.notion.site/GitHub-PRs-7112d03a6c4346168090b29a970c0154) and [here](https://github.com/mui/material-ui/blob/master/CONTRIBUTING.md#sending-a-pull-request).