xk6-disruptor: add a first-steps monopage

[roobre]

This PR continues the work started on https://github.com/grafana/k6-docs/pull/1356, creating a single-page aimed to onboard people into trying the disruptor with as little friction as possible.

The included examples borrow some explanation from other pages, introducing a bit of redundancy as the price to pay to avoid too many cross-references.

The examples use and reference QuickPizza as a demo application.