[GIT-PULLS] [php-src] PR #23373: docs: Internals Book improvements (06)

Pull Request: https://github.com/php/php-src/pull/23373
Author: NickSdot

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

This is the first PR that changes actual `docs-old` contents in their new locations: streams and output buffering. It removes A) clearly outdated/obsolete information B) parts that are not "internals" but userland. Diff to 05: https://github.com/NickSdot/php__php-src/compare/docs/restructure-05...docs/restructure-06

**Important:** 
naturally I can not know all details myself; I used Codex to help me to classify these old doc contents. I validated what it proposed and this seems good to me. However, this is the first PR where I explicitly would need feedback from people with more experience and historical knowledge -- a deeper look at what was removed and changed would be much appreciated!

### Preview
Branch: https://nicksdot.github.io/docs-restructure-06/
Final: https://nicksdot.github.io/docs-restructure-final/