feat: add treefmt-nix to dev flake

[gabyx]

Things Done

Closes: #707

• Added treefmt setup in dev flake to format treewide all files.
• Needed to update nixpkgs since treefmt-nix needs a newer input.
• Flake check in dev with nix flake check runs the formatting check. (Should be integrated in omnix by default?)
• Replaced nixpkgs-fmt with treefmt on pre-commit.
• .prettierrc.yaml in root (could be maybe somewhere else ...) to format all markdown files consistently.

How it works:

• just format will format all files: Once happy with the setup one can format the files on main in one commit. The docs folder with the markdown files look well treated by prettier and there is not much to change I guess.

Note: When merging, one should apply just format on main directly afterwards as chore: formatting all files treewide

Assisted-by: no agent.

[gabyx]

@shivaraj-bh: Could you maybe run just format on that branch and check how it looks?

[shivaraj-bh]

@gabyx Why use treefmt-nix when pre-commit has a nixfmt hook?

[gabyx]

@shivaraj-bh : Cause running hooks and formatting are two orthogonal things. Running hooks is fine but having one global process which formats treewide (with caching) is something which is not super easy achievable with hooks.

Running the format step (the exact same treefmt build and not again another nixfmt hook) in pre-commit is however the better option IMO.
Also you dont need to worry about other formats, its covered all.

Treefmt is awesome btw :). Once used you will probably deploy it everywhere :) (We did so and its a game changer :))