Add markdown checks (and fix some markdown issues)

[koppor]

We have Markdown files, where linting issues and wrong links could be contained. This PR adds a check for it. And also fixes issues of it.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

Is it possible to auto-fix or auto format those files after an edit? (especially when using github for editing)

[koppor]

Should we really automatically add a new commit to PRs? -- Fixing some linting issues is possible. Fixing broken links not.

I would not do that, because it's uncommon (isn't it?).

Similar discussion for checkstyle versus Refaster

[koppor]

Removed the link check. Will add it in a follow-up PR. Everything else is working now. Had to fix some more linting issues.