feat: Enhance membership section with CTA button

Updated membership information with pricing details and added
a call-to-action button linking to the join page. This aims to
increase engagement and improve user experience by providing
clear steps for potential new members.

templates/membership.html

@@ -23,9 +23,11 @@
             public.
           </p>
           <p class="card-text">
-            If you're interested in joining our association, get in touch with
-            us for more information.
+            Membership starts at € 5 / month!
           </p>
+          <a href="https://pcof.fi/join" class="btn btn-primary">
+            Join us now!
+          </a>
         </div>
       </div>
     </div>