Doc: Intro to architecture of web3/blockchain apps

[mDuo13]

Add an article explaining the general architecture of blockchain apps and explain the paradigm shift from the predominant patterns of past stuff.

Mainly, that a typical web3 app commonly involves a frontend (like a web app, running in-browser in JavaScript) that connects directly to the blockchain (or some API server with blockchain data), in contrast to older paradigms where you would have an application or backend server running PHP or Java or something, which in turn connects to a database (whether SQL or not).

In some ways the blockchain paradigm is a further evolution of internet architecture:

1. On-premises servers (run on your own computer)
2. Cloud computing (someone else runs it on their computer)
3. Blockchain (you don't even care whose computer is running it)

That's oversimplified because of course you still sort of care—both for efficiency and security—but a blockchain design abstracts away some of those concerns.

Also touch on some of the design implications and use cases because of how blockchain works: you can save private data locally on-device, and you can have some amount of public data on-chain, but it's wasteful and often inappropriate to put a lot of data into a blockchain, which is immutable and public by nature.

(Note: I expect to write this one but I'm not assigning it to myself because I'm not actively working on it yet.)