Restructure site to data-driven pages

Goal: make adding pages/items easy.

- [ ] Move full pages into `pages/`
- [ ] Move partials to `_includes/partials`
- [ ] Add `_data/navigation.yml` and loop in `header.html`
- [ ] Convert Catalogs and Tools to `_data/{catalogs,tools}.yml`
- [ ] Update pages to render from data

**Acceptance:** New items appear by adding a YAML row and merging a PR.