docs: add reference index pages

- add index pages for each reference documentation category
- move concepts pages into the reference hierarchy
- render clanModules overview page in the style of the CLI overview

docs/site/reference/index.md

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+# Documentation
+
+This section of the site contains information about the following topics:
+
+- How to use the [Clan CLI](./cli/index.md)
+- Available services and application [modules](./clanModules/index.md)
+- [Configuration options](./clan-core/index.md) controlling the essential features
+- Descriptions of the [Nix interfaces](./nix-api/index.md) for defining a Clan

docs/site/reference/nix-api/buildclan.md

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+# buildClan
+
+The core [function](https://git.clan.lol/clan/clan-core/src/branch/main/lib/build-clan/default.nix) that produces a Clan. It returns a set of consistent configurations for all machines with ready-to-use secrets, backups and other services.
+
+## Inputs
+
+`directory`
+: The directory containing the machines subdirectory
+
+`machines`
+: Allows to include machine-specific modules i.e. machines.${name} = { ... }
+
+`meta`
+: An optional set
+
+: `{ name :: string, icon :: string, description :: string }`
+
+`inventory`
+: Service set for easily configuring distributed services, such as backups
+
+: For more details see [Inventory](./inventory.md)
+
+`specialArgs`
+: Extra arguments to pass to nixosSystem i.e. useful to make self available
+
+`pkgsForSystem`
+: A function that maps from architecture to pkgs, if specified this nixpkgs will be only imported once for each system.
+  This improves performance, but all nipxkgs.* options will be ignored.
+  `(string -> pkgs )`

docs/site/reference/nix-api/index.md

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+# Nix API Overview
+
+There are two top-level components of the Nix API, which together allow for the declarative definition of a Clan:
+
+- the [Inventory](./inventory.md), a structure representing the machines, services, custom configurations, and other data that constitute a Clan, and
+- the [`buildClan`](./buildclan.md) function, which constructs a Clan from an Inventory definition.

docs/site/reference/nix-api/inventory.md

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+# Inventory
+
+`Inventory` is an abstract service layer for consistently configuring distributed services across machine boundaries.
+
+The following is the specification of the inventory in `cuelang`
+
+```cue
+{
+    meta: {
+        // A name of the clan (primarily shown by the UI)
+        name: string
+        // A description of the clan
+        description?: string
+        // The icon path
+        icon?: string
+    }
+
+    // A map of services
+    services: [string]: [string]: {
+        // Required meta fields
+        meta: {
+            name: string,
+            icon?: string
+            description?: string,
+        },
+        // Machines are added via the avilable roles
+        // Membership depends only on this field
+        roles: [string]: {
+            machines: [...string],
+            tags: [...string],
+        }
+        machines?: {
+            [string]: {
+                config?: {
+                    ...
+                }
+            }
+        },
+
+        // Global Configuration for the service
+        // Applied to all machines.
+        config?: {
+            // Schema depends on the module.
+            // It declares the interface how the service can be configured.
+            ...
+        }
+    }
+    // A map of machines, extends the machines of `buildClan`
+    machines: [string]: {
+        name: string,
+        description?: string,
+        icon?: string
+        tags: [...string]
+        system: string
+    }
+}
+```