docs: Install libxkbcommon (#16897)

This PR installs the development packages for `xkbcommon` and
`xkbcommon-x11` that are needed for building the `docs_preprocessor`.

Release Notes:

- N/A

.github/workflows/deploy_cloudflare.yml

@@ -24,6 +24,11 @@ jobs:
       - name: Set up default .cargo/config.toml
         run: cp ./.cargo/collab-config.toml ./.cargo/config.toml
 
+      - name: Install system dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install libxkbcommon-dev libxkbcommon-x11-dev
+
       - name: Build book
         run: |
           set -euo pipefail