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/agda.section.md

@@ -194,7 +194,7 @@ mkDerivation {
   version = "1.5.0";
   pname = "iowa-stdlib";
 
-  src = "...";
+  src = <...>;
 
   libraryFile = "";
   libraryName = "IAL-1.3";

doc/languages-frameworks/chicken.section.md

@@ -69,12 +69,12 @@ let
       chickenEggs = super.chickenEggs.overrideScope' (eggself: eggsuper: {
         srfi-180 = eggsuper.srfi-180.overrideAttrs {
           # path to a local copy of srfi-180
-          src = "...";
+          src = <...>;
         };
       });
   });
 in
 # Here, `myChickenPackages.chickenEggs.json-rpc`, which depends on `srfi-180` will use
 # the local copy of `srfi-180`.
-"..."
+<...>
 ```

doc/languages-frameworks/java.section.md

@@ -96,7 +96,7 @@ let
   something = (pkgs.something.override { jre = my_jre; });
   other = (pkgs.other.override { jre = my_jre; });
 in
-  "..."
+  <...>
 ```
 
 You can also specify what JDK your JRE should be based on, for example

doc/languages-frameworks/python.section.md

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