Setup CI/CD workflows

[ligurio]

As Travis CI is dead I propose to setup GH Actions with CI/CD where:

• automatically generate Sphinx docs per push
• publish HTML files generated by Sphinx to gh-pages on tag push (?)
• run tests (see appropriate section in .travis.yaml) with
  • lua 5.1
  • lua 5.2
  • lua 5.3
  • lua 5.4
  • luajit (latest)
  • tarantool 1.10
  • tarantool 2.[2-8]
• run doc tests to test examples from documentation
• run luacheck (?)