Doxygen cleanup.

- Add subpages definitions in appropriate
  section (User or Dev docs) for doc files
  (*.rst, *.md) that being listed at the top
  level of the generated doc page.
- Generate and add API list to RELEASE doc.
- Fix list_api_changes script to use HEAD
  as the endtag so it doesn't need to be
  changed every release.

Change-Id: Iace7b6433359c6b96869cb1db01facbbcb0ac1e6
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>

doxygen/user_doc.md

@@ -7,6 +7,7 @@ Several modules provide operational, dataplane-user focused documentation.
 - @subpage avf_plugin_doc
 - @subpage bfd_doc
 - @subpage dpdk_crypto_ipsec_doc
+- @subpage dhcp6_pd_doc
 - @subpage flowprobe_plugin_doc
 - @subpage ioam_plugin_doc
 - @subpage ipsec_gre_doc
@@ -16,10 +17,13 @@ Several modules provide operational, dataplane-user focused documentation.
 - @subpage lldp_doc
 - @subpage map_doc
 - @subpage marvel_plugin_doc
+- @subpage mtu_doc
 - @subpage nat64_doc
+- @subpage nat_ha_doc
 - @subpage qos_doc
 - @subpage selinux_doc
 - @subpage span_doc
 - @subpage srmpls_doc
 - @subpage srv6_doc
 - @subpage vcl_ldpreload_doc
+- @subpage vmxnet3_doc