[Docs] Create conceptual modules for the Custom Repositories

The Custom Repos documentation will contain an introductory abstract to inform what users can expect, what they need to know and what they will get from performing the steps in the procedural sections.

Please, refer to https://docs.google.com/document/d/1vK6mAIeqX2UTAijKnlHNSuPxp73-A3SKSWZ8DB5uOT0/edit#heading=h.ch7h01guy7zo

" chapter 2 conceptual text, we should explicitly mention users can define new repositories or modify an existing one. Also, the headings for 2.1 and 2.2 will accordingly change."

Ingrid and Loraine made some observations and suggestions as well. Thank you!