docs: add pregel conceptual doc (#3516)

* Update API Reference for Pregel
* Add conceptual page for Pregel
* The content for the two is very similar at the moment (i.e.,
duplicated content). This is usually a bad sign, but in this case I'm OK
duplicating information along both paths since the underlying algorithm
sets us apart from other implementations.

docs/docs/reference/pregel.md

@@ -1,9 +1,7 @@
-::: langgraph.pregel.Pregel
+# Pregel
+
+::: langgraph.pregel
     options:
       members:
-        - stream
-        - astream
-        - invoke
-        - ainvoke
-        - update_state
-        - aupdate_state
+        - Pregel 
+        - PregelNode