Setup docs deployments with mdBook (#11369)

This PR sets up deployments for the docs using mdBook.

Right now the new docs are hosted at
[zed.dev/docs2](https://zed.dev/docs2/).

The docs are deployed to Cloudflare Pages on merges to `main`, and we
have a Cloudflare Worker that routes traffic from `zed.dev/docs2` to the
docs deployment.

We can iterate on the docs for a bit, and then promote them to
`zed.dev/docs` when we're all ready for the switchover.

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

docs/book.toml

@@ -1,6 +1,7 @@
 [book]
-authors = ["Nate Butler"]
+authors = ["The Zed Team"]
 language = "en"
 multilingual = false
 src = "src"
-title = "Zed App Docs"
+title = "Zed"
+site-url = "/docs2/"