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

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