Add about submenu to the membership page

content/about/membership/contents.lr

 section: About
 ---
-section_id: membership
+section_id: about
 ---
 html: membership.html
 ---

templates/membership.html

@@ -128,4 +128,3 @@
   <h2 class="bg-white text-primary text-center p-4 mx-auto my-0">Become a Member</h2>
   <p class="bg-white text-center p-4">Join the Tor Project Membership Program and demonstrate your commitment to privacy online and become more deeply involved in the Tor community. Email us at <mark>giving@torproject.org.</mark> to get started.</p>
 </div>
-</div>