CmdStan User's Guide - document new method "laplace_sample"

#### Summary:

Create user-facing docs for CmdStan method `laplace_sample`,

#### Description:
The method `laplace_sample` uses Laplace approximation to generate a sample given estimates for all model paramters.

#### Additional Information:
Added to CmdStan via PR https://github.com/stan-dev/cmdstan/pull/1128.


#### Current Version:
v2.30.0
