Documentation: document iwd and connman in the NixOS manual

[gilice]

Problem

NetworkManager and wpa_supplicant
are documented in the NixOS manual but iwd and connman are not.

Proposal

Add an iwd section based on iwd.nix
and a conman section based on connman.nix to improve discoverability.

I intend to do this, but if someone beats me to it, that's fine.

Checklist

• checked latest Nixpkgs manual (source) and latest NixOS manual (source)
• checked open documentation issues for possible duplicates
• checked open documentation pull requests for possible solutions

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Note for maintainers: Please tag this issue in your PR.

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