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/nix-api/buildclan.md

@@ -0,0 +1,29 @@
+# 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 )`