Big simplification

This removes:

* Discussion of UI (`nix`) changes. The `nix` CLI is experimental
  anyway so no need to discuss it in an RFC.

* Flake references. These are mainly of use in the CLI. In flake.nix,
  we can use attrsets instead (e.g. `{ type = "github"; owner =
  "NixOS"; repo = "nixpkgs"; }` instead of `github:NixOS/nixpkgs`).

* The well-known outputs. The flake mechanism doesn't care about what
  the outputs are, only tools (like `nix`) care about particular
  outputs.

* Most of the discussion about registries. There are still indirect
  flake inputs (e.g. `nixpkgs`) but how these are resolved is an
  implementation detail of tools that update the lock file.