AST: Nodes for title should be shared (`admonitionTitle`, `cardTitle`)

[rowanc1]

We currently have admonitionTitle, cardTitle and a number of nodes that use admonitionTitle that are not admonitions. I think that these nodes should be title as a node type, perhaps with a kind. And then be shared between admonitions, dropdowns, topics, exercises, proofs, cards, etc.

[agoose77]

I'll touch this next week!

[rowanc1]

@agoose77 -- if you are good with this, I can update #1132 and can leave this one for you!