[php-src] PR #23374: docs: Internals Book improvements (07)

Pull Request: https://github.com/php/php-src/pull/23374 Author: NickSdot Depends on: #23368 (01), #23369 (02), #23370 (03), #23371 (04), #23372 (05), #23373 (06) This PR gets the extensions contents from docs-old in order. It removes obsolete and clearly userland targeting stuff which is not relevant to the internals book. Diff to 06: https://github.com/NickSdot/php__php-src/compare/docs/restructure-06...docs/restructure-07 **Important:** Personally I think unbundled extensions have nothing todo with the internals book. It should live in the actual PHP Docs. However, it was part of the docs-old and I didn't want to decide on my own whether to remove the "Unbundled Extensions" page or not. A decision is needed. ### Preview Branch: https://nicksdot.github.io/docs-restructure-07/ Final: https://nicksdot.github.io/docs-restructure-final/