Add .xrefcheck.yaml config to xrefcheck repo

[Martoon-00]

Clarification and motivation

Normally, any repository using xrefcheck should have a config, especially if it requires some exclusions to be added in order to operate correctly. And now we suffer from this repository itself lacking the config: whenever running xrefcheck we have to supply many --ignore options:

xrefcheck --ignore 'tests/markdowns/**/*' --ignore 'tests/golden/**/*' --ignore 'docs/output-sample/**/*'

I initially wanted to avoid having a config as that would require an extra maintenance effort. But disregard whether this concern is valid or not, after #156 there should be no problem with maintaining the config in our repository.

Acceptance criteria

• A small .xrefcheck.yaml is added to our repository, it should define only options that are necessary or very desired to have set custom.
• Running xrefcheck without any CLI options succeeds in master.
• CI commands are updated to avoid passing unnecessary command line options when calling xrefcheck, this includes the following CI configs:
  • .buildkite/pipeline.yml
  • .github/workflows/ci.yml