Add comprehensive inline documentation to all modules and enhance README

Co-authored-by: dwinkler1 <22460147+dwinkler1@users.noreply.github.com>

templates/rde/overlays/rix.nix

@@ -1,4 +1,21 @@
 # Rix overlay for R packages from rstats-on-nix
+#
+# This overlay provides access to R packages from the rstats-on-nix project.
+# rstats-on-nix maintains snapshots of CRAN packages built with Nix.
+#
+# Purpose:
+#   - Provides reproducible R package versions
+#   - Ensures binary cache availability for faster builds
+#   - Maintained by the rstats-on-nix community
+#
+# The rpkgs attribute gives access to:
+#   - rpkgs.rPackages: All CRAN packages
+#   - rpkgs.quarto: Quarto publishing system
+#   - rpkgs.rWrapper: R with package management
+#
+# Update the R snapshot date in flake.nix inputs section:
+#   rixpkgs.url = "github:rstats-on-nix/nixpkgs/YYYY-MM-DD"
 inputs: final: prev: {
+  # R packages from rstats-on-nix for the current system
   rpkgs = inputs.rixpkgs.legacyPackages.${prev.stdenv.hostPlatform.system};
 }