[php-src] PR #23371: docs: Internals Book improvements (04)

Pull Request: https://github.com/php/php-src/pull/23371 Author: NickSdot Depends on: #23368 (01), #23369 (02), #23370 (03) This is the start of integrating/removing the docs-old into the Internals Book (ref: https://github.com/php/php-src/pull/13338#issuecomment-5285942133). The PR deliberately only moves the old docs to their new locations without content changes. The next PRs will make actual edits -- the separation, again, is made to have a comparable diffs instead of deleted/added files. Diff to 03: https://github.com/NickSdot/php__php-src/compare/docs/restructure-03...docs/restructure-04 ### Preview Here: https://nicksdot.github.io/docs-restructure-04/ Final: https://nicksdot.github.io/docs-restructure-final/