Add warning category to broken link checker

[david-venhoff]

Motivation

For some pages the link checker shows an error, even though the page technically works for users. For this reason, a warning category should be added.

Proposed Solution

Add a warning category which should include the following links:

• Links to pages with status code 3xx
• Links with invalid fragment
• Links to pages with status code 403 because the page might just not be available to the link checker, even though it works for users.