Rewrite READMEs of non-autograded challenges

[carletex]

in BuidlGuidl/SpeedRunEthereum-v2#248, we moved the non-autograded challenges to the Build section (as builds ideas)

As Pablo mentions here: BuidlGuidl/SpeedRunEthereum-v2#249

In the Readme from our challenges without autograding, we could explain at the beggining with a nice informative format:
"This guide will walk you through creating an [NFT/Multisig Wallet]. Feel free to use it as inspiration to build your unique version of [NFT/Multisig Wallet] with your personal touch. If you're proud of the result, we encourage you to upload it to your profile builds to share it with the community".

I think we should reframe those READMEs a bit (less of a guided challenge and more like a build idea). We can start with a basic iteration.