treewide: Switch markdown placeholder from "..." to <...>

We use angle brackets since they look a lot like a placeholder while
also being valid nix code, as suggested by roberth here: https://github.com/NixOS/nixpkgs/pull/299554#discussion_r1541797970

doc/languages-frameworks/python.section.md

@@ -1890,7 +1890,7 @@ this snippet:
 {
   myPythonPackages = python3Packages.override {
     overrides = self: super: {
-      twisted = "...";
+      twisted = <...>;
     };
   };
 }