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>

README.md

@@ -227,7 +227,10 @@ named in that language:
 ```
 
 Besides records there are **listings**, pages this site composes itself —
-the tech radar at `/radar`, `/en/radar`, `/ca/radar` (`LISTINGS`). A panel page
+the tech radar at `/radar`, `/en/radar`, `/ca/radar`, and the ways to help at
+`/colabora`, `/en/contribute`, `/ca/col-labora` — the calls' own section, so it
+lists what `/colabora/{slug}` are, next to good first issues and a guide to a
+first contribution (`LISTINGS`). A panel page
 with the same slug would be shadowed by one, so they are few and deliberately
 named. Radar logos sit on a light tile in both themes: a brand's mark is not
 ours to recolour, and some (Rust's, black) would vanish in dark mode.