[php-src] PR #23372: docs: Internals Book improvements (05)

Pull Request: https://github.com/php/php-src/pull/23372 Author: NickSdot Depends on: #23368 (01), #23369 (02), #23370 (03), #23371 (04) This PR moves the two testing pages to a new hierarchy in preparation for the next PRs that reorganise the contents into a structure that can grow without crowding unrelated parts. As with the PRs before, this exists solely to have a helpful diff in the next PR. Diff to 04: https://github.com/NickSdot/php__php-src/compare/docs/restructure-04...docs/restructure-05 ### Preview Branch: https://nicksdot.github.io/docs-restructure-05/ Final: https://nicksdot.github.io/docs-restructure-final/