docs(jellyfin): Add some basic docs about Jellyfin

docs/src/apps/jellyfin.md

+# Jellyfin
+
+In the Shivering-Isles Infrastructure Jellyfin provides the media platform for streaming series and alike from the local NAS. Replacing services like Netflix for enjoying TV-Shows and Movies ripped from good old DVDs and Bluerays.
+
+Jellyfin itself is integrated with the Kubernetes cluster its running on using the [Jellyfin PDB Manager](https://git.shivering-isles.com/shivering-isles/infrastructure-gitops/-/tree/b919bd3d22d9b14cba5a64340720ef35aed5fa14/apps/base/jellyfin/jellyfin-pdb-mgr), that automatically configures the Pod Disruption Budget of the Jellyfin Pod to disallow disruptions while something is playing, to make sure maintenance work does not interfere with the watching experience.
+