The docpad-plugin-related
plugin is all about getting the related documents on an individual document which is not what you want. You don't need a plugin to do what you want.
Assuming your documents all have a tags
property, you can get a list of documents that match a specific tag (e.g. webdesign
) with code on your index page like this:
<ul>
<% for doc in @getCollection('documents').findAll({tags: '$in': `webdesign`}).toJSON(): %>
<li><a href="<%= doc.url %>"><%= doc.title %></a></li>
<% end %>
</ul>
If you want tag specific indexes for all of your tags, you probably want to look at docpad-plugin-tagging
(link).