IBX-4929: PHP API reference

[adriendupuis]

Question	Answer
JIRA Ticket	IBX-4929
Versions	master, 4.x, 3.3
Edition	Commerce (All)

• Generate and serve a PHP API reference
• Links to this PHP API reference instead of GitHub

TODO:

• [ ] The whole generated reference has been updated and committed (see #1972).

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