docs: Add npm install step

Author: adi-ray

src/content/contributor-docs/en/contributing_to_the_p5js_reference.mdx

@@ -382,6 +382,12 @@ The p5.js repository is set up so that you can generate and preview the referenc
 
 To do so, make sure you have committed your changes to a branch of your fork of p5.js.
 
+* First, ensure you have the necessary dependencies installed in the p5.js-website repository:
+
+```
+npm install
+```
+
 * Then, in the p5.js-website repo, run the following command, using the URL of your fork of p5 before the `#`, and the name of your branch after the `#`:
 
 ```