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

[email protected] (NickSdot)
Newsgroups php.git-pulls
Message-ID <[email protected]> 
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/
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