feat: introduce github actions to generate HTML and PDF automatially

[yumetodo]

Motivation

Sometimes, it's a bother for developers to install docker to local env.

@VFR-maniac is one of such developer.

https://twitter.com/ChaosSoulLover/status/1240244707197734912

Why don't you use CI? Let's use Github Actions!

Now, you don't need to install docker to local.

What changes did you make

• Write Github Actions config file to generate HTML and PDF and deploy to gh-pages branch
• fix build/gen.sh: some hack is now not required!

When CI will work?

only when you push to master branch.

What is required after you merge this PR?

You need to enable gh-pages.

see peaceiris/actions-gh-pages: GitHub Actions for GitHub Pages 🚀 Deploy static files and publish your site easily. Static-Site-Generators-friendly. #First Deployment with GITHUB_TOKEN

example deployed gh-pages

• https://yumetodo.github.io/opus-dash/opus-iso-binding.html
• https://yumetodo.github.io/opus-dash/opus-iso-binding.pdf

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[yumetodo]

@googlebot I signed it!

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