Add simple absorbing barrier example in the documentation with the extension method

After #74 we can follow the pattern of the code to solve it with an absorbing barrier.

The example to do is the identical one as the cleaned up example for the extension, except that you want to change the B and b For this,

[x] Keep the reflecting barrier at the top and put an absorbing barrier exactly at the bottom. (Resolved by b65416e)
[x] Have a new parameter called S which is the payoff if it ever hits the bottom. (Resolved by b65416e)
[x] Write down the same ODE as we are solving from https://quantecon.github.io/SimpleDifferentialOperators.jl/dev/#Solving-HJBE-with-constant-drifts-1 but then show the boundary conditions: \partial_x v(1) = 0 and v(x) = S for all x \leq 0 (Resolved by b65416e)
[x] Make the minimal changes to the extension code from thee #74 example to solve this. Basically just changing the one row in the b and the B. (Resolved by b65416e)

QuantEcon / SimpleDifferentialOperators.jl