mkdocs.yml: user_doc …/master → …/latest

[adriendupuis]

Question	Answer
JIRA Ticket
Versions	master
Edition

Target latest user doc version as master is hidden. (More consistent with reverse link.)

• dev master → user latest: #2633
• dev 4.6 → user 4.6: #2618
• dev 3.3 → user 3.3: #2619
• user master → dev latest: OK
• user 4.6 → dev 4.6: ibexa/documentation-user#335
• user 3.3 → dev 3.3: N/A

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