feat: filesystem routing for reports

[oliverlambson]

i want to handle

pages/
├── more/
│   ├── test.md
│   ├── test2.md
│   └── even-more/
│       ├── wow.md
│       └── thats-nested.md
├── populations.md
├── price-elasticity.md
└── vega-lite-is-cool.md

todo:

• get it to work
• style the index (make the <details> thing a bit nicer)
• update docs