[Docs] Create guide on how to reference entities

[jeffcampbellmakesgames]

Is your feature request related to a problem? Please describe. A topic that often comes up on Discord is how to reference Entities in other places like components or systems. This page here on the original wiki covers some areas. I'd like to create a similar page that covers some of the commonly recommended practices with example code provided:

• Storing reference to an Entity ID in a component
• Use a an EntityIndex to be able to search for zero or more entities based on other data.