Advanced start a network tutorial

[JoshOrndorff]

The existing private network tutorial walks the learner through starting a 2-node poa network using the node template. It teaches many important skills, but also leaves some questions.

This tutorial should walk through the process of starting a PoS network using the full substrate node (/bin/node). This is a useful exercise becuase there are several differences from the existing tutorial.

• Uses the session pallet
• More nodes - minor difference
• Requires initial stakes
• More session keys

The process

• build the node and subkey (no messing around with generating keys through Apps)
• Generate keys
• make a custom spec
  • Keys in session pallet
  • leave consensus pallets blank
  • initial stakes
• Start the network with three validators
• Start fourth node as non-validating full node
• Bond a fourth validator
• Insert keys into the fourth node and observe that it starts authoring.

I wrote some useful tidbits for this exercise in this support issue paritytech/substrate#5078