docs: add a contributing guide and link it from the README and site

Nothing in the repo said that pull requests go to develop rather than master,
or that release-plz reads the commit subject to pick the next version. Both
cost a first-time contributor a round trip.

CONTRIBUTING.md covers those, the three checks CI runs, and the fact that the
e2e suites skip without a soulfind server. The README and the community page
link to it, and it joins the footer's Project column.

website/src/components/layout/footer.tsx

@@ -20,6 +20,7 @@ const COLUMNS: readonly FooterColumn[] = [
     links: [
       { label: "GitHub", href: LINKS.gh },
       { label: "Issues", href: LINKS.issues },
+      { label: "Contributing", href: LINKS.contributing },
       { label: "Releases", href: LINKS.releases },
       { label: "Changelog", href: LINKS.changelog },
     ],