Add a link to the onboarding guide to the header

data/global.yaml

 link: [
   "./docs/index.html",
-  "https://gitlab.cosma.dur.ac.uk/swift/swiftsim"
+  "https://gitlab.cosma.dur.ac.uk/swift/swiftsim",
+  "./onboarding.pdf",
 ]
 
 navbar: [

templates/helpers.html

@@ -34,6 +34,9 @@
 
     <div class="right-nav" id="right-nav">
         <ul>
+            <li class="btn-yellow">
+                <a href="{{ link[2] }}"><i class="fa fa-book" aria-hidden="true"></i> Get Started !</a>
+            </li>
             <li class="btn-blue">
                 <a href="{{ link[0] }}"><i class="fa fa-book" aria-hidden="true"></i> Documentation</a>
             </li>