fix: remove deprecated lib.mdDoc

[Jappie3]

See the following for more info:

• [Tracking] Remove all uses of lib.mdDoc NixOS/nixpkgs#300735 for the tracking issue
• nixos/doc: dedocbookify NixOS/nixpkgs#237557 for the changes in the manual
• treewide: remove lib.mdDoc NixOS/nixpkgs#303841 removes all lib.mdDoc uses (13k changes)
• lib: add mdDoc back in to provide proper warning; set explicit timeline for removal NixOS/nixpkgs#304277 adds mdDoc back, fixes the warning & sets an explicit timeline for the final removal:

lib.mdDoc will be removed from nixpkgs in 24.11. Option descriptions are now in Markdown by default; you can remove any remaining uses of lib.mdDoc.

[Jappie3]

oh and here's a discourse post of people reporting that lib.mdDoc is broken
https://discourse.nixos.org/t/nix-darwin-is-broken/43497

[nikstur]

Thx!