Add view filters / pivots

[alerque]

Even with only a handful of examples and typesetters it is already becoming a mess. Some CSS layout and styling will help, but we're going to need some fundamentally different organization. I think each sample is going to need to be a stand alone "unit" that could be viewable either on its own or included in a list. Then we can have views of the gallery either by typesetter (e.g. show me all the Groff samples) or by sample (e.g. show me all the Hello Wold examples in every typesetter) or even some combination with filters, e.g. show me all the samples that are tagged 'math' in either Typst or SILE.