Add new docset: Nix functions (#5567)

Author: aiotter

docsets/Nix_Functions/README.md

@@ -0,0 +1,27 @@
+# Nix Functions Docset
+
+Author: Yuto Oguchi ([@aiotter](https://github.com/aiotter))
+
+## Contents
+
+* `builtins`
+* `pkgs.lib`
+
+## Generation steps
+
+```bash
+$ nix run github:aiotter/dash-nix-manual#generate-docset-functions
+```
+
+## Bug reports
+
+Please file issues at <https://github.com/aiotter/dash-nix-manual>.
+
+## Attributions
+
+- “NixOS Logo” by [Simon Frankau](https://github.com/simon-frankau),
+    [Tim Cuthbertson](https://github.com/timbertson),
+    and [Daniel Baker](https://github.com/djacu)
+    (maintained by the [NixOS Marketing Team](https://nixos.org/community/teams/marketing/)),
+    from [nixos/branding](https://github.com/NixOS/branding),
+    licensed under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).

docsets/Nix_Functions/docset.json

@@ -0,0 +1,11 @@
+{
+    "name": "Nix functions",
+    "version": "1.0.1",
+    "archive": "Nix_functions.tgz",
+    "author": {
+        "name": "Yuto Oguchi",
+        "link": "https://github.com/aiotter"
+    },
+    "aliases": ["nix", "nixpkgs"],
+    "specific_versions": []
+}