IBX-4175: Add links checker

[adriendupuis]

Question	Answer
JIRA Ticket	IBX-4175
Versions	N/A
Edition	N/A

Add a script to test internal and external links.

• Warn about broken links (not found, redirections, privates, etc.)
  • External links shouldn't be redirection (a redirection might be a not accurate enough replacement after a removal)
• Check if the fragment/anchor/hash exists (could be the symptom of moved content from one page to another)

Checklist

• [ ] Text renders correctly
• [ ] Text has been checked with vale
• [ ] Description metadata is up to date
• [ ] Redirects cover removed/moved pages
• [ ] Code samples are working
• [ ] PHP code samples have been fixed with PHP CS fixer
• [ ] Added link to this PR in relevant JIRA ticket or code PR