Refactor extensions list / categories XML entities

[joshtrichards]

This PR refactors the XML entities page used for the extensions categories pages (https://www.php.net/manual/en/extensions.php / etc).

Key changes:

• reformatted the code for easier maintenance (and incorporated PHP Style guidelines)
• improved introductory paragraphs for each category (that hopefully offering additional clarity)
• swapped ordering of State page sections (emphasizing experimental first followed by deprecated)

There are other follow-ups pending as I get time.

[lacatoire]

Suggested change

	but require require external libraries to be enabled.
	but require external libraries to be enabled.

[lacatoire]

Hey @joshtrichards, extensions.ent was removed by #5697, with its content migrated to entities/entities.extcat.ent. Would you be interested in porting your changes over there? The richer membership descriptions and the experimental-first ordering in the State section would still be worth having.

[lacatoire]

Closing for now, happy to reopen if you get back to the port.