Add systemd migration doc to pages list

mkdocs.yml

@@ -12,6 +12,7 @@ pages:
         - 4. LDAP (Optional): 'installation/4-ldap.md'
         - Upgrading NetBox: 'installation/upgrading.md'
         - Migrating to Python3: 'installation/migrating-to-python3.md'
+        - Migrating to systemd: 'installation/migrating-to-systemd.md'
     - Configuration:
         - Configuring NetBox: 'configuration/index.md'
         - Required Settings: 'configuration/required-settings.md'