fix: Add CNAME file to preserve custom domain on docs deployment

Added docs/CNAME file with 'moneyflow.dev' to prevent mkdocs gh-deploy
from overwriting the custom domain configuration.

Without this file, mkdocs gh-deploy --force removes the CNAME file from
the gh-pages branch, breaking the custom domain and reverting to
wesm.github.io/moneyflow.

mkdocs automatically copies docs/CNAME to the gh-pages branch during
deployment, preserving the custom domain configuration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: wesm

docs/CNAME

@@ -0,0 +1 @@
+moneyflow.dev