I guess if they were just tags, then just using regular tags related to the topic would be good, but since there's some arbitrary link between tags and the page navigation (haven't figured it out yet), maybe it needs to be consistent?
IIUC (which may not be the case :-) the navigation stuff (shown as tabs on the current template) comes from namespaces, ie pages with names foo:bar:thepage name. This is a docuwiki builtin.
Summary pages like howtos and snippets use the respective tags and a plugin to generate the pages. This plugin also generates the tagcloud on the front page. Add tags by {{tag>space separated list of tags}} which will tag the page with space, separated, list, of and tags :-)
Also, is there a better way for navigation than tags or however it works now? I found it very unintuitive to get a page listed in whatever the navigation part is using. Maybe there's a plugin that does a TOC (besides sitemap) based on links or something.
Maybe you just need to try again with this new info (assuming I'm right?)
Cheers Lex