It's not that difficult to add [[Category:Tools]] to the bottom of an article about SDS Drills for example or [[Category:Electrical]] to one about wiring a plug.
Then construct a template with all the top level categories in it for display on the "main page" and at the top of each category listing page (these are built automagically and include sub-category listings).
Most of the automatic pages aren't really much use like the Allpages or Categories one, the latter because it treats all categories as being at the same level.
and then lead them to indexes and/or search methods.
But it's not an "introduction" it's just a list of articles. Nothing about why or where those articals have come from.
The basics appears to be there already, it just needs some sorting out. The 50 odd categories just need something (a template?) to bring them into a more manageable/structred list form for display. The main categories Plumbing, Heating, Electrics, Lighting, Building etc already exist.