[ticket/12723] Add Sniff ensuring PHP files use the correct file header
* Nicofuma/ticket/12723:
[ticket/12723] Do not mention too many details on documentation.
[ticket/12723] Do not reference the coding guidelines section.
[ticket/12757] Add a Code Sniffer ruleset for PHP files of phpBB extensions
[ticket/12723] Properly describe new Code Sniffer rulsets.
[ticket/12723] Use core suffix in filename instead of phpbb in the middle.
[ticket/12723] Add Sniff ensuring PHP files use the correct file header