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

@@ -0,0 +1,8 @@
+# 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