frontpage: add npm documentation links (#22173)

Co-authored-by: Laura Coursen <lecoursen@github.com>

content/index.md

@@ -55,4 +55,9 @@ externalProducts:
     name: CodeQL
     href: 'https://codeql.github.com/docs'
     external: true
+  npm:
+    id: npm
+    name: npm
+    href: 'https://docs.npmjs.com/'
+    external: true
 ---

tests/rendering/sidebar.js

@@ -34,6 +34,7 @@ describe('sidebar', () => {
     expect(
       $homePage('[data-testid=sidebar] a[href="https://codeql.github.com/docs"]')
     ).toHaveLength(1)
+    expect($homePage('[data-testid=sidebar] a[href="https://docs.npmjs.com/"]')).toHaveLength(1)
   })
 
   test('adds `data-is-current-page` and `data-is-active-category` properties to the sidebar link for the current page', async () => {