docs: add docs directory info and nixos options improvement plan

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/nixos-improvements.md

@@ -2,6 +2,26 @@
 
 This document contains planned improvements to the NixOS infrastructure that are not directly part of the automated deployment pipeline.
 
+## Planned
+
+### Custom NixOS Options for Service and System Configuration
+
+Currently, most service configurations in `services/` and shared system configurations in `system/` are written as plain NixOS module imports without declaring custom options. This means host-specific customization is done by directly setting upstream NixOS options or by duplicating configuration across hosts.
+
+The `homelab.dns` module (`modules/homelab/dns.nix`) is the first example of defining custom options under a `homelab.*` namespace. This pattern should be extended to more of the repository's configuration.
+
+**Goals:**
+
+- Define `homelab.*` options for services and shared configuration where it makes sense, following the pattern established by `homelab.dns`
+- Allow hosts to enable/configure services declaratively (e.g. `homelab.monitoring.enable`, `homelab.http-proxy.virtualHosts`) rather than importing opaque module files
+- Keep options simple and focused — wrap only the parts that vary between hosts or that benefit from a clearer interface. Not everything needs a custom option.
+
+**Candidate areas:**
+
+- `system/` modules (e.g. auto-upgrade schedule, ACME CA URL, monitoring endpoints)
+- `services/` modules where multiple hosts use the same service with different parameters
+- Cross-cutting concerns that are currently implicit (e.g. which Loki endpoint promtail ships to)
+
 ## Completed
 
 - [DNS Automation](completed/dns-automation.md) - Automatically generate DNS entries from host configurations