Categorisation of pages

[thopanX]

I propose a categorisation feature to cluster wiki pages by topic. This way, one could create topical landing pages automagically after categorising pages.

[Nutomic]

Instances are meant to work as sort of categories, for example articles about open source should be on ibis.wiki. A category setting would also make sense, but its not clear who would define the available categories. Lemmy had hardcoded categories a long time ago, but those were removed because they werent flexible enough. And if admins can define categories its not clear how it would federate.

Similar to #152