docs: migrate third-parties documentation to a dedicated folder (#2068)

* docs: migrate third party documentation to a dedidcated folders

---------

Signed-off-by: Ludovic Ortega <ludovic.ortega@adminafk.fr>

docs/getting-started/third-parties/nixpkg.mdx

@@ -0,0 +1,17 @@
+---
+title: Nix Package Manager (Advanced)
+description: Install Jellyseerr using Nixpkgs
+sidebar_position: 1
+---
+
+import { JellyseerrVersion, NixpkgVersion } from '@site/src/components/JellyseerrVersion';
+import Admonition from '@theme/Admonition';
+import Tabs from '@theme/Tabs';
+import TabItem from '@theme/TabItem';
+
+# Nix Package Manager
+:::warning
+Third-party installation methods are maintained by the community. The Seerr team is not responsible for these packages.
+:::
+
+Refer to [NixOS documentation](https://search.nixos.org/options?channel=25.05&query=seerr)