Add markdown linter and link validator
Created by: maxwolfs
In order to implement the standards and their lifecycle state to https://docs.scs.community, it's necessary that all markdown files are linted correctly and in the same way.
This PR:
- adds the same link validator action and markdownlint with ruleset as in the doccs repository
- removes old markdownlint action
Additionally to linting all markdown files I have added the missing frontmatter prop title
to some of the standards.