🤖 Fix cmux.io custom domain deployment (#49)

Fixes cmux.io 404 error by ensuring CNAME file is included in
deployment.

## Problem

- cmux.io returns 404
- GitHub Pages custom domain requires a CNAME file in the root of
deployed files
- mdbook builds to `docs/book/` directory, which becomes the root when
deployed
- The CNAME file was not being included in the build output

## Why This Fixes It

GitHub Pages uses the CNAME file to configure custom domain routing:

1. When GitHub Pages deploys, it looks for a CNAME file in the root of
the deployed site
2. If present, it configures the custom domain to serve that content
3. Without it, the custom domain (cmux.io) has no content to serve → 404

Our workflow:
- Builds docs with mdbook → outputs to `docs/book/`
- Uploads `docs/book/` as artifact → this becomes the root of the
deployed site
- **Problem**: CNAME file was in `docs/` but not copied to `docs/book/`
- **Solution**: Copy CNAME to build output so it's included in
deployment

## Changes

- Create `docs/CNAME` with 'cmux.io'
- Add workflow step: `cp docs/CNAME docs/book/CNAME` after build
- Ensures CNAME is in the deployed artifact root

## Testing

Once merged and deployed, https://cmux.io should resolve correctly.

_Generated with `cmux`_

Author: ammario

.github/workflows/docs.yml

@@ -42,6 +42,9 @@ jobs:
       - name: Build docs
         run: cd docs && mdbook build
 
+      - name: Copy CNAME to build output
+        run: cp docs/CNAME docs/book/CNAME
+
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3
         with:

docs/CNAME

@@ -0,0 +1 @@
+cmux.io