Add Topics Section

We need a topics sections to actually collate all the packages in a useful form - see https://www.arewewebyet.org/topics/

We can probably add this as a section in Publish and then have a page per section for each topic

Start by reviewing the existing Publish section and the linked arewewebyet.org/topics/ example to understand the current structure. Define how topics and their package lists should be represented, then add the section and topic pages so each package is collated under a useful topic and the navigation reaches them.

Written by the indexing model from the issue text.