docs: update prefix two-digit number in routing section (#77758)

## Description

The current [routing
documents](https://github.com/vercel/next.js/tree/canary/docs/01-app/03-building-your-application/01-routing)
are missing `01` and `02` prefix.

This PR changes the numbering of them and update [contribution-guide
docs](https://github.com/vercel/next.js/blob/canary/docs/04-community/01-contribution-guide.mdx).

### Improving Documentation

- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution
follows the docs guidelines:
https://nextjs.org/docs/community/contribution-guide

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## For Contributors

Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>

Author: Marukome0743

docs/04-community/01-contribution-guide.mdx

@@ -79,17 +79,17 @@ For example, in the [functions API Reference](/docs/app/api-reference/functions)
 └── ...
 ```
 
-But, in the [routing section](/docs/app), the files are prefixed with a two-digit number, sorted in the order developers should learn these concepts:
+But, in the [app router section](/docs/app), the files are prefixed with a two-digit number, sorted in the order developers should learn these concepts:
 
 ```txt
-01-routing
-├── 01-defining-routes.mdx
-├── 02-pages.mdx
-├── 03-layouts-and-templates.mdx
+01-getting-started
+├── 01-installation.mdx
+├── 02-project-structure.mdx
+├── 03-layouts-and-pages.mdx
 └── ...
 ```
 
-To quickly find a page, you can use `⌘ + P` (Mac) or `Ctrl + P` (Windows) to open the search bar on VSCode. Then, type the slug of the page you're looking for. E.g. `defining-routes`
+To quickly find a page, you can use `⌘ + P` (Mac) or `Ctrl + P` (Windows) to open the search bar on VSCode. Then, type the slug of the page you're looking for. E.g. `installation`
 
 > **Why not use a manifest?**
 >