[GIT-PULLS] [php-src] PR #23375: docs: Internals Book improvements (08)

Pull Request: https://github.com/php/php-src/pull/23375
Author: NickSdot

Depends on: ##23368 (01), #23369 (02), #23370 (03), #23371 (04), #23372 (05), #23373 (06), #23374 (07)

The "Writing Tests" page was 1:1 moved from QA in https://github.com/php/php-src/pull/18409. It is too large for a single page; hard to navigate and the sidebar navigation bloated. Separating sections and samples (and later more) makes it much easier to navigate and digest. Diff to 07: https://github.com/NickSdot/php__php-src/compare/docs/restructure-07...docs/restructure-08

**Note:**
This change is the reason why I started improving the Internals Book in the first place. As some already noticed, I am trying to make myself helpful with testing and QA related stuff. The testing docs are dated, incomplete, and not exactly easy to digest. This change allows me to improve the testing docs in a growable structure as I go over the next weeks. Think of: coverage docs, updating examples and samples, general refresh and formatting, adding requirements, etc. etc. So this is not the "final" or perfect state, but the foundation for what comes, and already a step forward to what we have now. 🫡

### Preview 

(both same, unless another branch/PR after 08 will added)

Branch:  https://nicksdot.github.io/docs-restructure-08/
Final: https://nicksdot.github.io/docs-restructure-final/

### Aside
Unless a discussions in any of the PRs require to add another follow up PR to make the chain mergable this is the last PR of this series. Everything else can be normal PRs that do not depends on previews changes.