Update CI documentation

[embg]

Travis CI has ended free usage for OSS. Additionally, the instructions for setting up Travis (and possibly the other services) are outdated. Since we have moved almost all of our CI onto Github Actions (which don't require any setup for forks), I think we should only require Github Actions to pass before submitting PRs against the main repo.

[Cyan4973]

Looks fine for the general message,

but it seems we are missing an opportunity to educate our users.
The previous version of CONTRIBUTING.md was providing detailed instructions about how to subscribe to external CI services, in order to increase test coverage. In the proposed update, they are simply gone, with no replacement.

I could imagine that some of these instructions have become obsolete, and some CI may no longer be possible (TravisCI notably). But they are not all useless. We shouldn't go as far as discouraging users from adding other CI beyond Github Actions.

To counter the risk of obsolescence of local doc, try to find a documentation from the CI provider itself, and link to it.
The link might also die over time, though it's an acceptable limit to what it's reasonable to provide.