Task: create a contributing guideline for documentation

In order for contributors to write new material more effectively, add a lightweight guideline for creating (and reviewing) contributions to documentation.

What:

• make sure the documentation category is chosen appropriately
  • for tutorials: https://nix.dev/contributing/writing-a-tutorial
• make sure new material is placed sensibly:
  • https://github.com/NixOS/nix.dev/blob/master/CONTRIBUTING.md
• check the writing guides:
  • https://nix.dev/contributing/documentation#guidelines
• make sure you understand what the new/changed documentation tries to tell you
  • user testing is one thing, the reviewer can also play the role of a new user

Where to put it:

• https://nix.dev/contributing/documentation#guidelines
  • expand this section by prepending the instructions
  • move list of available documentation and communication channels into a sub-file

Start with the documentation guidelines section at nix.dev/contributing/documentation and read CONTRIBUTING.md to understand placement conventions. Add the contribution and review guidance there, and move the documentation and communication channels list into a sub-file; done means the guidelines are discoverable and the existing links remain usable.