Switch from Travis and Appveyor to GitHub Actions

[vitaut]

Travis CI has been slow recently and Appveyor has always been a disaster. Look into switching CI to GitHub Actions.

[vitaut]

Mostly done: https://github.com/fmtlib/fmt/actions. Just need to migrate the documentation build config.

[alexezeder]

GitHub Actions also can be used to automate releasing a new version. Looks like ChangeLog.rst always has changes for the desired release, so a special workflow for version tags handling can be created. It would create a new release from the version tag, copy release description from ChangeLog.rst and upload an archive asset. The uploaded archive will be the same as it is for existing releases: with only necessary files for end-users (maybe fewer files than right now), prepared documentation, etc.

I'm sure that it's automized already by some service, script, or by something else, but work with releases on GitHub can be automized by using GitHub, which is nice.

[vitaut]

The documentation build is now working: https://fmt.dev/dev/. The main remaining thing is to add testing of newer standards.

[vitaut]

Added more standards in f8640d4.