IBX-9609: Strict validation

[adriendupuis]

Question	Answer
JIRA Ticket	IBX-9609
Versions	All
Edition	All

Get a strict validation as recommended in https://www.mkdocs.org/user-guide/configuration/#validation following #2627

For master & 4.6, it needs

• ~~#2665~~ Just ignored
• #2669
• IBX-9578 fix of #dynamic-settings-injection

For 3.3, it needs

• #2667
• #2668

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

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