Lots of rewriting for the Scala 3 Reference

[Sporarum]

Big to small rewritings of the Scala 3 Reference

Mostly yo make it more clear or to improve structure, and fix typos
But there are some specs that were out of date, notably tupling/untupling

[Sporarum]

Motivation removed because it was already in the main article (parameter-untupling)

[bishabosha]

and identical content

[Sporarum]

@bishabosha
I'll let you review one last time derivation.md, if you accept we can merge
The rest shouldn't have changed since your last review

The derivation.md is not complete, I'll open a different PR to go more into details

[Sporarum]

The PR is here: #16170
(Maybe I should have waited to open it not to use CI resources)

[bishabosha]

@Sporarum this needs a rebase, but I have some other points - perhaps this should be split into two PRs.

1. based on language-reference-stable that only contains changes valid for 3.2.0 - this means changes will be published immediately to docs.scala-lang.org
2. based on main that adds everything else, these will be published only on dotty.epfl.ch, and then docs.scala-lang.org when 3.2.2 comes out

hopefully it should all be ok to base on the stable branch (as current nightly is still 3.2.x) - but beware of experimental stuff added in patch releases

[Sporarum]

@Sporarum this needs a rebase, but I have some other points - perhaps this should be split into two PRs.

1. based on `language-reference-stable` that only contains changes valid for 3.2.0 - this means changes will be published immediately to docs.scala-lang.org

2. based on `main` that adds everything else, these will be published only on dotty.epfl.ch, and then docs.scala-lang.org when 3.2.2 comes out

hopefully it should all be ok to base on the stable branch (as current nightly is still 3.2.x) - but beware of experimental stuff added in patch releases

In the end, everything can be rebased onto language-reference-stable without issue

This PR has been change to point to language-reference-stable as well

@bishabosha Do you think we can merge it ?

changes still to come