Document lambdas (#5300)

Moved everything from "Syntax Overview" through "Self and Recursion" in
https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3848.md
to docs/design and added the link to it in README.md

Still needs a short summary for the README

Closes #4898

---------

Co-authored-by: josh11b <[email protected]>

Author: IvanDuran02

docs/design/README.md

@@ -3793,7 +3793,11 @@ the critical underpinnings of such abstractions.
 
 #### Lambdas
 
-> **TODO:**
+> **TODO:** References need to be evolved. Needs a detailed design and a high
+> level summary provided inline.
+
+> References: [Lambdas](lambdas.md),
+> [Proposal #3848: Lambdas](https://github.com/carbon-language/carbon-lang/pull/3848)
 
 #### Co-routines