docs: add docs for clanLib

docs/site/reference/clanLib/index.md

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+`clanLib` is a collection of functions and utilities
+
+!!! Danger
+    unless explicitly mentioned everything in ClanLib is for internal use in clan-core.
+
+## Stable Attributes
+
+The following attributes are publicly maintained
+
+### `clanLib.clan`
+
+A function that takes [clan options](../../reference/options/clan.md)
+The option definitions can be composed by means of `imports`
+
+Returns an evaluated clan configuration - A `lib.evalModules` result:
+
+- `.config`: *The main result*
+- `.options`,`.moduleGraph`, ...: For debugging.