Add more advanced examples

[Michał Stęchły]

It would be great to have some more advanced examples of routines written in QART.

Two that come to mind are:

• Alias Sampling – add it to the documentation as a "more advanced example"
• Double Factorization – too much for the docs, but would be great reference for anyone who wants to do sth more with QART

We could also consider adding all the other subroutines from Double Factorization, so people have some building blocks to start playing with.

[Michał Stęchły]

Here's a working file with Alias Sampling:
alias_sampling.json