Tutorial: Add custom node keys in the Chainsspec

[DrW3RK]

The solochain and parachain templates use Alice/Bob keys as the validator/collator keys for simplicity. There should be examples of how these keys can be replaced with custom keys either through ChainSpec as Invulnerables or as candidates through the pallet

Some existing docs with this content https://hackmd.io/@s_iGZLIITG6WjSgnFX0pcg/the-collator-setup-guide

https://web3educhain.xyz/collator/#setting-up-collators-in-the-chain-spec

https://docs.substrate.io/tutorials/build-a-blockchain/add-trusted-nodes/