Clarify with commas (#31224)

Adds a few commas where they could improve the clarity of the sentence.

- [X] Make sure the linting passes by running `yarn lint`


Co-authored-by: Tim Neutkens <6324199+timneutkens@users.noreply.github.com>

docs/routing/introduction.md

@@ -6,7 +6,7 @@ description: Next.js has a built-in, opinionated, and file-system based Router.
 
 Next.js has a file-system based router built on the [concept of pages](/docs/basic-features/pages.md).
 
-When a file is added to the `pages` directory it's automatically available as a route.
+When a file is added to the `pages` directory, it's automatically available as a route.
 
 The files inside the `pages` directory can be used to define most common patterns.
 
@@ -19,14 +19,14 @@ The router will automatically route files named `index` to the root of the direc
 
 #### Nested routes
 
-The router supports nested files. If you create a nested folder structure files will be automatically routed in the same way still.
+The router supports nested files. If you create a nested folder structure, files will automatically be routed in the same way still.
 
 - `pages/blog/first-post.js` → `/blog/first-post`
 - `pages/dashboard/settings/username.js` → `/dashboard/settings/username`
 
 #### Dynamic route segments
 
-To match a dynamic segment you can use the bracket syntax. This allows you to match named parameters.
+To match a dynamic segment, you can use the bracket syntax. This allows you to match named parameters.
 
 - `pages/blog/[slug].js` → `/blog/:slug` (`/blog/hello-world`)
 - `pages/[username]/settings.js` → `/:username/settings` (`/foo/settings`)
@@ -68,7 +68,7 @@ function Home() {
 export default Home
 ```
 
-In the example above we have multiple links, each one maps a path (`href`) to a known page:
+The example above uses multiple links. Each one maps a path (`href`) to a known page:
 
 - `/` → `pages/index.js`
 - `/about` → `pages/about.js`