[polaris.shopify.com] Update markdown file structure (#7393)

Removes the need for every markdown file to live in a separate
directory.

Before | After
---|---
`/foo/bar/index.md` | `/foo/bar.md`
`/baz/biz/index.md` | `/foo/biz.md`

### Why?
- More intuitive IMO
- Easier to jump to the right file in VSCode
- To create a page, you only need to add a markdown file, not a
directory + markdown file
- Maps to how .dev organizes their content

### Validating that it works

1. I've clicked through most pages / templates to make sure they work

2. I've clicked through search results to make sure they work

3. `sitemap.xml` has the same number of entries in both `main` and this
branch

4. I've run a diff check on `nav.json` between `main` and this branch.
The only diff is a capitalization mistake that I fixed.
<img width="914" alt="Screen Shot 2022-10-13 at 3 33 12 PM"
src="https://user-images.githubusercontent.com/875708/195611763-94e58b1f-338b-466f-8734-652ecb443e47.png">

5. I've run a diff check on `site.json` between `main` and this branch.
The only diff is a capitalization mistake that I fixed.
<img width="918" alt="Screen Shot 2022-10-13 at 3 33 19 PM"
src="https://user-images.githubusercontent.com/875708/195612211-cfd793eb-7fc8-4fb9-b895-d8e5959a6d99.png">

Co-authored-by: Gui Santos <guilherme.santos@shopify.com>

Author: martenbjork