Add guides/fullnode

[NunoAlexandre]

This PR:

• Changes the format to separate CLI argument setup from the runtime setup, making the guide more readable.
• Added monitoring options and checks
• Reviewed the guide for more clarifying language
• Added more bootnodes for redundancy
• Added demo testnet

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[NunoAlexandre]

@gpmayorga would be awesome to have your feedback here when you can. I will run through the guide myself to double check no step is outdated or wrong. I think the node options are now outdated due to to the update to 0.9.43 so I am expecting changes there but hopefully the rest is mostly in place 👍

[gpmayorga]

This PR is ready to go !

[gpmayorga]

All suggestions addressed, Ready to go @wischli and @hieronx

[hieronx]

Included in https://github.com/centrifuge/documentation/pull/493