Use Polkadot binary instead of building for all parachain examples

[nuke-web3]

Building polkadot is not needed when binaries exist in release assets: https://github.com/paritytech/polkadot/releases/ And docker images are available: https://github.com/paritytech/polkadot/blob/master/doc/docker.md

This will save time for most users, and they can always compile it themselves if needed.

[liamhession]

I'd like to help here, which would start with me understanding better what changes this calls for.

1. Would the success criterion be to update the docs so that in each relevant how-to guide, the reader would be encouraged to a) download the binary and run it, or b) download the docker image and run it, instead of building Polkadot from scratch locally? And a short explanation of how to do a & b would either be added to the guide or added elsewhere and linked to?
2. Is it true that the only examples you feel this time-saving approach is relevant to are the ones in the collection of "Parachains" how-to guides? If not, you can just say no and i can go through the docs to collect more how-to guides outside that set for which this approach is relevant