Add GitHub Actions Workflow for Continuous Deployment of HTML Documents

[ATQlove]

What

I have added a new GitHub Actions workflow file named cd-convert-debian-bookworm.yml. This workflow automates the continuous deployment (CD) process, including converting documents using TeXmacs and deploying the results to the gh-pages branch.

Why

1. Through this CD implementation, HTML documents in the docs folder can be automatically deployed to the webpage via GitHub Pages.
2. TeXmacs requires a graphical environment to run. Since GitHub Actions runs in a headless environment, I included xvfb to simulate a display server, allowing TeXmacs to operate without a physical display.
3. The workflow is also triggered on pull requests to the main branch to demonstrate to reviewers that the file runs correctly. This trigger condition will be removed once the pull request meets the merge requirements.

[ATQlove]

@da-liii Please help me to see if there is something wrong with this pr, thanks!