Set up CI

[Lipen]

We need to set up a CI/CD pipeline with GitHub Actions.

Currently, we use https://latexonline.cc (repo: https://github.com/aslushnikov/latex-online) to render the main PDF, but it uses texlive2020, which is outdated and cannot compile documents with modern packages. For example, tabularray is very old (so is unusable), and hyperref has no unicode support in bookmarks.

Notes:

• We could test that the project is engine-independent, i.e. compiles with different latex engines: pdfLaTeX, XeLaTeX, tectonic.
• We could build the documents in different formats (ebook, a4, etc).
• Resulting PDF(s) (main, and maybe for each chapter) could be uploaded via upload-artifact action.