Use GitHub actions for continuous integration (CI)

[clue]

Bye bye Travis CI, you've served us well.

With these changes applied, the website will now build and deploy using GitHub actions. Unlike the previous Travis CI setup which used a number of built-ins, I've also updated the documentation to describe the manual deployment process in case this automatic behavior should ever break again.

This (manual) deployment process required a number of minor updates to make sure the deployed website behaves exactly like the original site. You're looking at ~8 hours of various attempts to get this to be identical to the previous deployment.

Refs reactphp/reactphp#442

[clue]

I've also set this up so the workflow can be triggered manually. This is particularly useful because we don't currently have a workflow that triggers a website rebuild when a new component release is tagged. For now, this can be triggered in the actions tab like this:

https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow

In the future, we should create additional workflows for each of our components that would then trigger a rebuild of the website deployment workflow (see also #44). I'll look into this together with @SimonFrings some time in the future once this PR is in.