Improve rewrites documentation (#34725)
Improve rewrites documentation (#34725)

This is a PR to update the rewrites documentation. This is after struggling to get rewrites to work with `trailingSlash` for a customer as this wasn't documented. The main culprit was the `:path*` wildcard not catching trailing slashes. The changes made to for this commit were:

- [x] Added example for redirecting to blog and docs
- [x] Expanded on original code example for external URLs
- [x] Added code example for using rewrites with `trailingSlash`

### Context

For a reproduction of the `:path*` wildcard not catching trailing slashes as expected, see below. Click on the "not working" demo link below, navigate to a specific blog post, then reload the page. It will redirect infinitely.

- [Not working demo of main repo with `:path*`](https://redirect-demo-git-not-working-nutlope.vercel.app/blog/)
- [Working demo of main repo with `:path(.+)`](https://redirect-demo.vercel.app/blog/)

If you want to take a look at the code, here are the full repos. They both have `trailingSlash: true`:

- [not working main repo](https://github.com/Nutlope/redirect-demo/tree/not-working) 
- [working main repo](https://github.com/Nutlope/redirect-demo)
- [blog repo](https://github.com/Nutlope/starter-blog-example)
JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js