Add source file linting for Markdown and YAML files

[kbabioch]

This implements linting for Markdown and YAML files within the repository as GitHub actions. Upon pushing the linters will be triggered, checking if there are any issues with the file formats.

This ensures more consistency and quality throughout the project.

The linters that are being used are:

• yamllint: https://github.com/adrienverge/yamllint
• markdownlint: https://github.com/DavidAnson/markdownlint

Some minor issues have already been identified with the existing files. Those are also being addressed with this pull request in order to make (and hopefully keep) the linters happy.

In addition to that there are configuration files (.yamllint.yml and .markdownlint.yml) that can be used to fine-tune the linters.

[kbabioch]

Most likely there will be some conflicts with #47. Depending on which one you're going to merge first, I'm happy to rebase it.

[Bouni]

Ye, as you expected, some merge conflicts 😄

[Bouni]

Awesome!