add workflow to validate yaml files

[andibraeu]

we maybe want to prevent corrupt html results to be published.

to make use of this, I guess branch protection has to be enabled and a required check validate-yaml should be configured

[coderbyheart]

I like the idea of validating the YAML files, however as you can see from your check there are a lot of false positives that do not actually cause an error.

I suggest to implement validation based on some content rules (for example using a JSON schema with the YAML files converted to JSON).

[andibraeu]

ok, that's why I set the level to warning for newline and trailing spaces. It is also possible to disable all these warnings

But let me try https://github.com/marketplace/actions/json-yaml-validate first