Document lambdas

[geoffromer]

Description of the bug:

We now have an approved design for lambdas (see #3848), but it isn't documented in the language design.

Link to documentation:

https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/README.md#lambdas

What should it say instead?

Everything from "Syntax Overview" through "Self and Recursion" in https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3848.md would be a good starting point, probably as a separate file under docs/design. Eventually there should be a short summary in README.md, but for now a link to the detailed doc is probably enough; the most important thing is for the design docs to be discoverable and up to date, so they can act as a source of truth.

Any other information, logs, or outputs that you want to share?

No response