Add Markdown Linter for Documentation

[mhellmeier]

🚀 Feature Request

Current Problem

The structure and format of the markdown files inside the documentation (Docusaurus) aren't specified.

Proposed Solution

A markdown linter should be used to check and improve the structure of the documentation. Further, a new GitHub workflow action is needed so that every change of a markdown file is directly checked in every pull request.

Additional Context

At the beginning, the currently available markdown linters must be checked and evaluated. Popular ones are:

• https://github.com/DavidAnson/markdownlint
• https://github.com/DavidAnson/markdownlint-cli
• https://github.com/DavidAnson/markdownlint-cli2
• https://github.com/markdownlint/markdownlint
• ...