Add another dev shell to `Toolchain/flake.nix` called `ladybird.nix`
that pulls in the dependencies for building Ladybird.
Also update the documentation to mention building with a flake.
Add a nix flake to `Toolchain/` that wraps the existing nix derivation
`Toolchain/serenity.nix`. This also comes with a lockfile making the nix
developer enviornment setup more reproducible.
We also update the documentation for "other builds" to mention the
flake.