docs: Internals Book improvements (01)

[NickSdot]

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:

• docs: Internals Book improvements (02) #23369
• docs: Internals Book improvements (03) #23370
• docs: Internals Book improvements (04) #23371
• docs: Internals Book improvements (05) #23372
  until here it was more or less mechanical changes; they should be seen as preparation for the following 3 that change actual old contents.
• docs: Internals Book improvements (06) #23373
• docs: Internals Book improvements (07) #23374
• docs: Internals Book improvements (08) #23375

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"; after this one I should be done with the bigger changes. So please kindly bear with me. :)

Related

#13338 (#13338 (comment))
#15939
#15940
#17761
#18409