Improve docs sidenav active link highlighting

Capture the slug once before applying it and add a slash to it to so we get a unique string to match against instead of a fuzzy partial 'contains'. Helps avoid 'grid' highlighting 'flexbox-grid', for example.

docs/_includes/nav-docs.html

@@ -25,6 +25,7 @@
       <ul class="nav bd-sidenav">
         {% for doc in group.pages %}
           {% assign slug = doc.title | downcase | replace: ' ', '-' | replace:'-&-','-' %}
+          {% capture slug %}/{{ slug }}{% endcapture %}
           {% assign active = nil %}
 
           {% if page.url contains slug %}