docs(www): point the website, docs and badges back at GitHub (#35)

* docs(www): point the website, docs and badges back at GitHub

GitHub is the release destination again, so every user-facing reference to the
self-hosted Gitea instance now points at github.com/AdrianBonpin:

- www: nav + footer links, Download / Releases buttons, 'View on GitHub' and
  'Star on GitHub' CTAs, landing.js download fallback label, the repo
  preconnect/dns-prefetch hints, and the ROADMAP link (Gitea's
  src/branch/prod path becomes GitHub's blob/prod)
- www/src/lib/seo.test.ts: matching structured-data fixtures
- README: shields.io badge and the tap prose
- ROADMAP: Homebrew tap URL

Deliberately left pointing at Gitea (they are the fallback, not destinations):
.gitea/workflows/release.yml and scripts/release-mac.sh use git.ranio.xyz as
an API host, so rewriting those would break the fallback path.

* fix(www): update the stale FAQ count assertion

faq.test.ts asserted exactly 5 FAQ items but the list has had 6 since
bdf2bb5 (Release 0.7.15), so the www suite has been red since then. No CI
runs the www tests, which is why it went unnoticed. Found while verifying the
docs change above; unrelated to it.

www/public/landing.js

@@ -59,12 +59,12 @@
     const isMobile = /Mobi|Android|iPhone|iPad|iPod/i.test(userAgent);
     const keepLabel = downloadBtn.hasAttribute("data-download-label");
 
-    const releasesUrl = "https://git.ranio.xyz/adrianbonpin/gridline/releases";
+    const releasesUrl = "https://github.com/AdrianBonpin/gridline/releases";
     const version = "0.8.0";
 
     if (isMobile) {
       downloadBtn.href = releasesUrl;
-      if (!keepLabel) downloadBtn.textContent = "Get it on Gitea";
+      if (!keepLabel) downloadBtn.textContent = "Get it on GitHub";
     } else if (isMac) {
       downloadBtn.href = `${releasesUrl}/download/v${version}/Gridline_${version}_aarch64.dmg`;
       if (!keepLabel) downloadBtn.textContent = "Download for macOS";