List and document experimental features in one place

Problem

Currently experimental features are listed in the setting experimental-features, but documented in detail on a rather explanation-style page aimed at contributors.

Proposal

The second should not have that listing at all because it's unrelated, and the listing in the setting should contain the details.

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

Priorities

Add :+1: to issues you find important.