Docs/update - Add implied step (#1144)

## What is this PR doing?
Missing a step - fix that

## What problem is it solving?
Helps new developers not get caught before starting

## How is the problem addressed?
Enter the directory, then install NPM packages

packages/docs/site/docs/11-architecture/18-host-your-own-playground.md

@@ -57,6 +57,11 @@ Create a shallow clone of the Playground repository, or your own fork.
 git clone -b trunk --single-branch --depth 1 [email protected]:WordPress/wordpress-playground.git
 ```
 
+Enter the `wordpress-playground` directory.
+```sh
+cd wordpress-playground
+```
+
 Install dependencies, and build the website.
 
 ```sh