[GIT-PULLS] [php-src] PR #23368: docs: Internals Book improvements (01)

Pull Request: https://github.com/php/php-src/pull/23368
Author: NickSdot

WIP

This is switching the internals book to Markdown. The PR is formatting only and deliberately:
- makes no content changes
- leaves file extension untouched

### Why Markdown?

- not yet another syntax contributors have to learn
- rendered correctly by default everywhere; including GitHub and IDEs
- keep diffs when moving/merging `docs-old`  clean
- because we can; there is an official plugin for exactly that
- we can easily extend it later if we want to

This is a preflight in preparation for:
- #NNN (02)
- #NNN (03) 
- #NNN (04)
- #NNN (05) -- until here it were more or less mechanical changes; they should be seen as preparation for the next 3.
- #NNN (06)
- #NNN (07)
- #NNN (08)

The work is split into multiple PRs to keep diffs comparable and focused; without splitting them git would mark quite literally everything as deleted and new files.

### Preview
Branch: https://nicksdot.github.io/docs-restructure-01/
Final: https://nicksdot.github.io/docs-restructure-final/

### Aside

I got multiple times asked to split my PRs into multiple smaller PRs. But I also got approached and told that it is annoying. Hence, I'd like to address that quick. Here splitting is essential because with the many moves _and_ content changes in one PR the diff would simply not be reviewable. My serial PRs were needed because things either depended strongly on each other or touched too many files/extensions for one PR. That said, I consider the work that I have been doing over the last few weeks as "groundwork", and after this one I should be done with the bigger changes. So please kindly bear with me. :)