Port ""Add "Exact mechanism" section to derivation.md"" to main

[Sporarum]

Branch fix-derivation-main was created as follows:

# Make sure main is up to date
git checkout fix-derivation
git checkout -b fix-derivation-main
git rebase --onto main language-reference-stable fix-derivation-main
# Make PR into main from fix-derivation-main

[bishabosha]

Did this method have any git conflict or just work?

[Sporarum]

Did this method have any git conflict or just work?

This time it had no conflict

Last time there were some conflicts but I seem to remember they were justified