add guide notebook (#8258)

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Co-authored-by: Nuno Campos <nuno@boringbits.io>
Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

docs/docs_skeleton/docs/guides/expression_language/index.mdx

@@ -0,0 +1,9 @@
+# LangChain Expression Language
+
+import DocCardList from "@theme/DocCardList";
+
+LangChain Expression Language is a declarative way to easily compose chains together.
+Any chain constructed this way will automatically have full sync, async, and streaming support.
+See guides below for how to interact with chains constructed this way as well as cookbook examples.
+
+<DocCardList />