update header

Author: tballmsft

_layouts/default.html

@@ -21,6 +21,7 @@ <h1 class="project-name">MicroCode - {{ page.title | default: site.title | defau
       <h2 class="project-tagline">{{ page.description }}</h2>
       {% endif %}
       <p>
+        <a href="https://{{ site.github.owner_name }}.github.io/{{ site.github.repository_name }}/docs/purchase">Purchasing</a> |
         <a href="https://{{ site.github.owner_name }}.github.io/{{ site.github.repository_name }}/docs/manual">User guide</a> |
         <a href="https://{{ site.github.owner_name }}.github.io/{{ site.github.repository_name }}/docs/language">Language</a> |
         <a href="https://{{ site.github.owner_name }}.github.io/{{ site.github.repository_name }}/docs/samples">Samples</a> |