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/pages/home.page.scss

@@ -40,6 +40,11 @@
   margin-top: 1.25rem;
   padding: 0;
   list-style: none;
+
+  // `@pi/ui`'s reset spaces every `li + li`; this list is spaced by `gap`.
+  > li {
+    margin: 0;
+  }
 }
 
 .landing-subsection {