[php-src] PR #23369: docs: Internals Book improvements (02)

Pull Request: https://github.com/php/php-src/pull/23369 Author: NickSdot Depends on: #23368 (01) This PR only makes the change from .rst to .md extensons. It's separated because otherwise the diff in the following PRs would classify files as deleted/added instead of showing the actual content changes (as this PR's diff proves). The diff of this PR (and the following) is useless until the previous PR is merged. So here are the actual changes of this PR agaist the previous branch: https://github.com/NickSdot/php__php-src/compare/docs/restructure-01...docs/restructure-02 ### Preview Branch: https://nicksdot.github.io/docs-restructure-02/ Final: https://nicksdot.github.io/docs-restructure-final/