Update guide and docs

[jsoares]

• The guide is supposed to go on the docs website, so it's here for review and awaiting docs merging
  • It's supposed to be a really quick path towards having something working, so it skips on the explanations and can't handle exceptions. But the goal is to have something that works reliably on a clean setup and allows for testing.
• The readme has been stripped to just build the agent and get it working with a rootnet, with everything else moving to subdocs
  • Also made an effort to simplify things across the board
  • These subdocs can stay here or move to the docs website in some form
• I tried to harmonise a number of things, e.g. code blocks, argument passing with = or without, etc. but probably didn't go all the way.
• I think I also replaced all ipc-agent commands with the new grouped variants but may have missed something
• The guide assumes that the default branches will work. That's not currently true so someone following the guide will fail. It will hopefully be true on Monday.

[jsoares]

One note: The subnet deployment instructions (under docs/subnet) are very duplicative for the single- and multi-node case... I think we should consider dropping the single node instructions; other than some minor simplifying aspects (e.g. sharing default wallet), it's just a special case of multinode.

[adlrocha]

This is great! Thank you @jsoares

[jsoares]

@adlrocha Made a number of improvements, merging anyhow as agreed once the checks finish I mark it as ready for review and can actually merge it 😊 Please tag me on the x-net message PR.

@TippyFlitsUK I ended up reorganising the readme as sort of an index for everything IPC usage, and moving the guide into the docs folder. We should figure out how to use the docs website (which is now out-of-date) and repo to complement each other.