Add GitHub Action to deploy docs to gh-pages on main branch commits

[yorkie]

Add a GitHub Action workflow that builds and deploys the project documentation site to the gh-pages branch on each commit to the main branch. The workflow should:

• Trigger on every push to the main branch
• Copy the contents of the docs directory
• Push the docs content to the gh-pages branch (overwriting its contents)

This will enable automatic deployment of the project website for each update to main.

Implementation notes:

• No build step is required; the docs directory should simply be published as-is.
• Ensure the gh-pages branch contains only the docs contents after deployment.
• Use appropriate GitHub Actions for branch push and directory sync.