Refine Compton Telescope Introduction

[ckarwin]

The Compton-telescope-data-analysis-intro was originally written for DC1. It has been edited to be more general, but it can probably still use some refinements. There is quite a bit of overlap with the description in the cosipy intro (which is fine), but I think it's good to keep them both. I see the Compton-telescope-data-analysis-intro as a very basic description of how Compton telescopes work and the corresponding data space, whereas the cosipy intro goes into more details, and also does so in the context of COSI.

So this issue is to refine the description in the Compton-telescope-data-analysis-intro, with the goal of providing a clear and simple explanation for someone who has little to no experience with Compton telescopes (i.e. a starting point).