Use continuous integration

[sebschrader]

We do already have a Docker-based development and testing environment and integration tests. Having Travis build Hades shouldn't be too hard.

[lukasjuhrich]

s/Travis/Github Actions/

A CI pipeline should also handle:

1. Basic python Linting (without the docker setup)
2. Building the docs in nit-picky mode (-n)
3. Building Hades (with the docker setup)

We can run (1/2, 3) in parallel.
Deployment of the docs by means of a push to the gh-pages branch is not important, but we should build the docs anyway

1. we can catch errors in the documentation earlier
2. we are forced to keep the sphinx build itself working (a dev could easily forget adding a plugin to the [docs] requirements after introducing it to the docs).