docs: improve smart contracts

[barriebyron]

https://docs.minaprotocol.com/zkapps/o1js/smart-contracts
These comments were embedded (now removed) in the Markdown comment for the smart contracts doc

Let's flesh out this page so it answers the most important questions for devs
TODOs
IMO, needs to be fleshed out a bit more before sending users to "test your zkApp" or "advanced topics".
Basics that are missing:

• payments
• Mina.transaction / at least hinting at the basic mechanics of how to compile / prove
• the general notion of a transaction / of an account update (party)?
• permissions? (probably an advanced topic)

more notes
TODO Gregor's note on the below alert box: too much? too early? I think little "deep dives" like this can be useful to answer questions that more advanced users often have after reading our docs, and spread more understanding of Mina to the internet

TODO: create at least a basic section about zk circuits below this subsection which contains (among other things) a more thourough explanation of what this tip hints at