doc: add documentation on monads

doc/SUMMARY.md

@@ -59,6 +59,15 @@
   - [Thunk](./thunk.md)
   - [Task and Thread](./task.md)
 - [Functions](./functions.md)
+- [Monads](./monads/intro.md)
+  - [Functor](./monads/functors.lean.md)
+  - [Applicative](./monads/applicatives.lean.md)
+  - [Monad](./monads/monads.lean.md)
+  - [Reader](./monads/readers.lean.md)
+  - [State](./monads/states.lean.md)
+  - [Except](./monads/except.lean.md)
+  - [Transformers](./monads/transformers.lean.md)
+  - [Laws](./monads/laws.lean.md)
 
 # Other
 

doc/images/monads.dgml

@@ -0,0 +1,55 @@
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+    <Link Source="StateM" Target="Monad" Bounds="-726.395530552052,-304.861622913356,30.7140523342301,66.8616229133556" />
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+      <Setter Property="Background" Value="#FF91E7ED" />
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doc/monads/.gitignore

@@ -0,0 +1 @@
+*.lean.md