docs: tell quickstart readers how to get past the npm edgesOut crash

`npm create @angular@21` fails with "Cannot read properties of null
(reading 'edgesOut')" on npm older than 11.6, which is what the
newest Node.js 20 and 22 releases come with. It is an npm bug
(npm/cli#9787), but a reader hits it at step 1.

The line says to delete the project folder because the failed run
leaves one behind, and running the command again into it fails with
a merge conflict on package.json.

Fixes #3785

Author: armando-navarro

docs/install-and-setup.md

@@ -35,6 +35,8 @@ cd <project-name>
 
 These commands set up an Angular 21 project, the version AngularFire 21 supports.
 
+If the npm commands stop with `npm error Cannot read properties of null (reading 'edgesOut')`, you have an npm older than 11.6, where installing a new Angular project crashes ([npm/cli#9787](https://github.com/npm/cli/issues/9787)). Run `npm install -g npm@11`, delete the project folder the failed command left behind, and run the command again.
+
 ### 2. Install AngularFire and Firebase
 
 AngularFire 21 is currently a release candidate, published under the `next` tag: