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

[armando-navarro]

Fixes #3785

Adds one line under step 1 of docs/install-and-setup.md for readers whose npm create @angular@21 stops with npm error Cannot read properties of null (reading 'edgesOut').

Verification

On 2026-10-02, with Node.js 22.23.2:

• npm create @angular@21 <project-name> on npm 10.9.8 fails with that error and leaves the project folder behind.
• With npm 11.21.0 the same command installs.
• Running it again into the leftover folder fails with A merge conflicted on path "/<project-name>/package.json"., which is why the line says to delete the folder first.
• A new Angular 21 project fails to install on npm 10.8.2, 10.9.8, 10.9.9, 11.3.0 and 11.5.2, and installs on 11.6.0, 11.19.0, 11.21.0 and 12.2.0.
• The line says npm@11 and not npm@latest because npm 12 requires Node.js 22.22.2 or later, and Angular 21 also supports Node.js 20.

[tyler-reitz]

Approving.

One to watch: npm/cli#9787 is still open and labeled "Cannot Reproduce", so the 11.6 boundary rests on your bisect rather than on a landed fix. It could move again in a later npm.

Not blocking, but worth considering: Node 20.20.2 ships npm 10.8.2 and Node 22.23.3 ships npm 10.9.9, both in your failing set. So every reader on a current Node 20 or 22 hits this at step 1, not an unlucky subset. That may justify making npm install -g npm@11 an unconditional instruction in step 1 rather than a recovery line people only read after it breaks.

[armando-navarro]

On the boundary, it turns out to be a landed fix: npm/cli#8448 closed npm/cli#8261, the same edgesOut error, and shipped in @npmcli/arborist 9.1.4, which npm 11.6.0 is the first to bundle. npm 10 is still on arborist 8, which is why 10.9.9 fails. npm/cli#9787 is the open report of it against npm 10.

I took your suggestion about making it unconditional. I moved it to "Before you begin" as a requirement for npm 11.6 or later, in the follow-up commit.