Migrate posts, pages, collections, and data, along with their layouts

[beechnut]

Move all page and post content to content/ folder.

Things that can be skipped:

• Page content doesn't have to be fully correct.
• Styles, images, and links can be broken — this work is scoped for future tickets
• Blog post pagination can be skipped. We have to migrate paginators.
• Additional functionality like shortcodes (Jekyll filters and plugins) can be stubbed or skipped for now — this task focuses on migrating the bulk of the content and having it write the correct pages/permalinks.

Acceptance criteria

Beta Give feedback

1. site builds
2. blog posts render with correct permalinks
3. pages (from 18f.gsa.gov pages/) all render with correct permalinks
4. products and services pages all render with correct permalinks
5. Page dates (<time> tag data) renders the same or acceptably close
6. authors render with correct permalinks (not in scope: turning author files into _data/authors.yml
7. There's no content/ folder in _site
8. Google verification (google16a62a069d0c4fa4) comes over correctly
9. Check robots.txt (present) and humans.txt (absent) files

[beechnut]

Factored out the remaining work into a separate checklist item in #3839

Closed by #3850