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>

.github/workflows/deploy_docs.yml

@@ -0,0 +1,35 @@
+name: Deploy Docs
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  deploy-docs:
+    name: Deploy Docs
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+        with:
+          clean: false
+
+      - name: Setup mdBook
+        uses: peaceiris/actions-mdbook@v2
+        with:
+          mdbook-version: "0.4.37"
+
+      - name: Build book
+        run: |
+          set -euo pipefail
+          mkdir -p target/deploy
+          mdbook build ./docs --dest-dir=../target/deploy/docs2/
+
+      - name: Deploy
+        uses: cloudflare/wrangler-action@v3
+        with:
+          apiToken: ${{ secrets.CLOUDFLARE_API_TOKEN }}
+          accountId: ${{ secrets.CLOUDFLARE_ACCOUNT_ID }}
+          command: pages deploy target/deploy --project-name=docs