Merge pull request #255128 from PuercoPop/ruby-document-extraConfigPaths

ruby: document extraConfigPaths option from bundlerEnv

doc/languages-frameworks/ruby.section.md

···
 mkShell { buildInputs = [ gems (lowPrio gems.wrappedRuby) ]; }
 ```
 
+Sometimes a Gemfile references other files. Such as `.ruby-version` or vendored gems. When copying the Gemfile to the nix store we need to copy those files alongside. This can be done using `extraConfigPaths`. For example:
+
+```nix
+  gems = bundlerEnv {
+    name = "gems-for-some-project";
+    gemdir = ./.;
+    extraConfigPaths = [ "${./.}/.ruby-version" ];
+  };
+```
+
 ### Gem-specific configurations and workarounds {#gem-specific-configurations-and-workarounds}
 
 In some cases, especially if the gem has native extensions, you might need to modify the way the gem is built.