software-origins/nixguix: Document the origins

Start documenting the page about nixguix [1]

[1] https://docs.softwareheritage.org/user/software-origins/nixguix.html

Refs. swh-loader-core#4749 (closed)

Refs. swh/meta#3781 (closed)

docs/software-origins-support.yml

@@ -324,7 +324,12 @@ forges:
     lister:
       status: prod
     loader:
-      status: N/A
+      status: prod
+      source: https://gitlab.softwareheritage.org/swh/devel/swh-loader-core/
+      package_name: swh.loader.core
+    metadata:
+      intrinsic: none
+      extrinsic: none
     grant: nlnet-tweag-2020
     developer: tweag