treewide: Fix all Nix ASTs in all markdown files

This allows for correct highlighting and maybe future automatic
formatting. The AST was verified to work with nixfmt only.

doc/languages-frameworks/dotnet.section.md

@@ -134,7 +134,7 @@ Here is an example `default.nix`, using some of the previously discussed argumen
 { lib, buildDotnetModule, dotnetCorePackages, ffmpeg }:
 
 let
-  referencedProject = import ../../bar { ... };
+  referencedProject = import ../../bar { /* ... */ };
 in buildDotnetModule rec {
   pname = "someDotnetApplication";
   version = "0.1";