docs: fix up the markdown

Type: docs
Change-Id: Ia541839e1f1ceddfae4579dece43b9cc820702e2
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
(cherry picked from commit de569048a5)

doxygen/dev_doc.md

@@ -19,4 +19,5 @@ Programming notes for developers.
 - @subpage handoff_queue_demo_plugin
 - @subpage lcov_code_coverage
 - @subpage mdata_doc
+- @subpage mempreload_doc
 

doxygen/user_doc.md

@@ -29,3 +29,5 @@ Several modules provide operational, dataplane-user focused documentation.
 - @subpage srv6_doc
 - @subpage vcl_ldpreload_doc
 - @subpage vmxnet3_doc
+- @subpage af_xdp_doc
+- @subpage wireguard_plugin_doc

src/plugins/wireguard/README.md

@@ -1,4 +1,4 @@
-# Wireguard vpp-plugin
+# Wireguard vpp-plugin  {#wireguard_plugin_doc}
 
 ## Overview
 This plugin is an implementation of [wireguard protocol](https://www.wireguard.com/) for VPP. It allows one to create secure VPN tunnels.