userguide: hide subpage links using "el" class, use plain MarkDown links

docs/userguide.md

@@ -2,9 +2,17 @@
 
 This User Guide is organized into the following sections
 
-- @subpage quickstart -- how to quickly start using pmp-library
-- @subpage installation -- detailed installation instructions
-- @subpage overview -- an overview of the library and its capabilities
-- @subpage tutorial -- how to perform basic tasks
-- @subpage contributing -- guidelines for contributions
-- @subpage codingstyle -- contains our coding guidelines
+- [Quickstart](quickstart.html) -- how to quickly start using pmp-library
+- [Installation](installation.html) -- detailed installation instructions
+- [Overview](overview.html) -- an overview of the library and its capabilities
+- [Tutorial](tutorial.html) -- how to perform basic tasks
+- [Contributing](contributing.html) -- guidelines for contributions
+- [Coding Style](codingstyle.html) -- contains our coding guidelines
+
+
+<span style="display: none;"> @subpage quickstart </span>
+<span style="display: none;"> @subpage installation </span>
+<span style="display: none;"> @subpage overview </span>
+<span style="display: none;"> @subpage tutorial </span>
+<span style="display: none;"> @subpage contributing </span>
+<span style="display: none;"> @subpage codingstyle </span>