docs: add section about idiomatic options

[ambroisie]

Description

Draft to work towards (finally) closing #6441.

I think this would belong into a larger section about common patterns in Home Manager, like the shell and VCS integration options. Feel free to suggest more patterns that should belong in this section.

Still a draft, as it needs to be actually written.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted nixf-diagnose --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News

[khaneliman]

This is a great idea for calling out the reusable logic we would prefer throughout the codebase.

[ambroisie]

Additional "don't": full paths to a binary in an environment variable (e.g: EDITOR), as it breaks on a profile upgrade + GC.