Allow exporting sections with markdown

[bitspittle]

Right now you can only use markdown to create pages, but I found a few places where I would have liked to use them for sections in a page (for example, writing instructions that appear in a pop-up).

Exact approach to solve this is still up for debate, but I think maybe you should be able to config wildcard rules in a markdown config block (right now, essentially "* -> com/example/mysite/pages") but imagine if we could add additional rules, like "sections/* -> com/examples/mysite/components/sections". Then, if a destination targets something that is not under the pages/ folder, it isn't exported with a @Page tag.

Setting to 1.0 milestone for now because it could be a great demo for Kobweb to lean on this. Also, maybe we'll want to do something backwards incompatibile. But push comes to shove, we can probably postpone to 1.1.