[php-src] PR #23370: docs: Internals Book improvements (03)

Pull Request: https://github.com/php/php-src/pull/23370 Author: NickSdot WIP Depends on: #23368 (01), #23369 (02) Some existing files have inline todos with links that 404. This PR is moving these into todo files (excluded from rendering) so that we no longer have broken internal links. Though, some of the todos are obsolete and/or handled later; this is a mechanical move without taking decisions on that. Diff to 02: https://github.com/NickSdot/php__php-src/compare/docs/restructure-02...docs/restructure-03 ### Preview Branch: https://nicksdot.github.io/docs-restructure-03/ Final: https://nicksdot.github.io/docs-restructure-final/