Add 'key concepts' page

[jbaross-pometry]

This page should provide a conceptual introduction to Raphtory assuming very minimal familiarity with graph/network analysis.

General topics:

• Graph concepts, graph, node, edge, layer
• What makes Raphtory different from others e.g. temporal stuff but also any unusual termonological choices we make

Raphtory topics:

• What is view?
• What is a nodestate ?
• What is collection and when to use it ?
• Generally how to work with Raphtory concepts - what does a flow diagram loop look like for an analyst
• rolling vs expanding
• edge vs exploded edge
• properties vs metadata
• History -> returns timeindex's which you then operate on