Add a page for every way to help

/colabora (/en/contribute, /ca/col-labora) — the calls' own section, so
it lists what /colabora/{slug} are — brings the open calls, the good
first issues across the collective's repositories, and a five-step
guide from picking a task to having it reviewed. With nothing open and
nothing labelled, the guide and the other ways to help are still there.
The landing's "contribute" section links to it.

Also fixes the landing's issue list, which @pi/ui's `li + li` margin
spaced twice over.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

landing/src/app/core/routes.spec.ts

@@ -108,6 +108,17 @@ describe('listings', () => {
       ca: '/ca/radar',
     });
   });
+
+  // The calls' section with nothing after it is the list of ways to help.
+  it('reads a call section on its own as the contribute listing', () => {
+    expect(resolveRoute('/colabora')).toEqual({
+      type: 'contribute',
+      locale: 'es',
+      slug: '',
+    });
+    expect(resolveRoute('/en/contribute')?.type).toBe('contribute');
+    expect(resolveRoute('/en/contribute/frontend')?.type).toBe('call');
+  });
 });
 
 describe('homeHref', () => {