Update markdown link checker to use linkspector github action

[hugobuddel]

In #9 we moved from markdown-link-spec to linkspector.

There was no github action for linkspector at the time, so we had to setup something ourselves. Now there is an 'official' github action, so we could move to that: https://github.com/UmbrellaDocs/action-linkspector

Also, there now is a default configuration file, so it is not necessary to create one: UmbrellaDocs/linkspector#14