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

CONTRIBUTING.md

···
 
   ```nix
   foo {
-    arg = "...";
+    arg = <...>;
   }
   ```
 
···
   ```nix
   foo
   {
-    arg = "...";
+    arg = <...>;
   }
   ```
 
   Also fine is
 
   ```nix
-  foo { arg = "..."; }
+  foo { arg = <...>; }
   ```
 
   if it's a short call.
···
 - Functions should list their expected arguments as precisely as possible. That is, write
 
   ```nix
-  { stdenv, fetchurl, perl }: "..."
+  { stdenv, fetchurl, perl }: <...>
   ```
 
   instead of
 
   ```nix
-  args: with args; "..."
+  args: with args; <...>
   ```
 
   or
 
   ```nix
-  { stdenv, fetchurl, perl, ... }: "..."
+  { stdenv, fetchurl, perl, ... }: <...>
   ```
 
   For functions that are truly generic in the number of arguments (such as wrappers around `mkDerivation`) that have some required arguments, you should write them using an `@`-pattern:

doc/build-helpers/fetchers.chapter.md

···
 buildPythonPackage rec {
   pname = "pysimplesoap";
   version = "1.16.2";
-  src = "...";
+  src = <...>;
 
   patches = [
     (fetchDebianPatch {

doc/languages-frameworks/agda.section.md

···
   version = "1.5.0";
   pname = "iowa-stdlib";
 
-  src = "...";
+  src = <...>;
 
   libraryFile = "";
   libraryName = "IAL-1.3";

doc/languages-frameworks/chicken.section.md

···
       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

···
   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

···
 {
   myPythonPackages = python3Packages.override {
     overrides = self: super: {
-      twisted = "...";
+      twisted = <...>;
     };
   };
 }