[DOCS] Replace the Receptor Tools README with a link to the Receptor Getting Started on RTD

[tvo318]

Replace the content of the entire README to provide a link to the Getting Started Guide in the RTD page.
This will eliminate overlapping content and content getting out-of-sync and provide one place for source of truth, which will be the RTD page. The graphic needs to be source-controlled using the drawing.io files exported to PNG.
Draft content of Getting started resides here: https://docs.google.com/document/d/1hGCfWE1U0ZFipzWdFO56A8jrCvAjoeYbaYjLKGpM1IQ/edit#heading=h.ol8cfa1l4vd2
@fosterseth @TheRealHaoLiu @thom-at-redhat

[tvo318]

Use DrawIO plugin to update the graphic: #886