Complete Umami event tracking coverage across all partials

Add data-umami-event attributes to all meaningful links that were
missing tracking:
- Footer: try-it, GitHub, docs, API, blog, newsletter, community,
  about, contact, and all social icons (Framagit, GitHub, feedback,
  Mastodon, email)
- CTA join section: primary signup button
- Usage footer: survey, Matrix, IRC, and about links (with per-page
  content base name for granular tracking)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

layouts/partials/cta-join.html

@@ -8,7 +8,7 @@
           {{ i18n "cta-account-text" }}
         </p>
       </div>
-      <a href="/join" class="btn btn-lg btn-light px-4" style="border-radius: 2em">{{ i18n "cta-account-button" }}</a>
+      <a href="/join" class="btn btn-lg btn-light px-4" style="border-radius: 2em" data-umami-event="cta-join">{{ i18n "cta-account-button" }}</a>
     </div>
   </div>
 </section>