Technical introduction to the XRP Ledger

[mDuo13]

The XRPL Overview doesn't provide enough of a technical introduction to how the XRPL really works. We should have a one- or two-page explainer that summarizes key parts of the XRP Ledger at a high level so that developers can get oriented quickly and have the context to understand other docs as needed.

The previous introduction could be a good starting place to look for content, though we want to do more than just reinstate it.