166: Nix formatting

Create 0101-nix-formatting.md

WIP

Go through a large part and agree on it

Co-Authored-By: @piegamesde

Update 0101-nix-formatting.md

Rework a lot of things

Update 0101-nix-formatting.md

Move around some sections

Reword the detailed section

Minor updates

Slight header changes again

Updates

Update 0101-nix-formatting.md

Update after today's meeting

Update 0101-nix-formatting.md

Further updates in the meeting

Update 0101-nix-formatting.md

After todays meeting

Update after meeting

Rename to probably the right number

Only use anchor links

Improvements and additions

- The sub-expression rule is now reworded and its own section with
  examples and rationale
- Line length limit is now specified as we agreed-upon in the meeting
- The operator section is rewritten to align more with the consensus

Redo and explain operator special case

Also remove the special case for non-chainable operators, barely any benefit
in Nixpkgs