what are some examples of well-designed protocol / platform doc sites?

some things to look for:
- are equally useful both for end users and developers
- are equally useful for new and advanced users
- make it easy to get started
- make it easy to find what you're looking for

The Polkadot's doc site is best I've seen regarding the metrics you mentioned: 
https://polkadot.network/development/docs/ covering everything from "end users's needs" to protocol specification to grants...

Building @warpcast and @farcaster, new dad, like making food

But the best eli5 docs I've ever seen are the "Canon's user manuals", literally explaining everything for the IQ of -70, or with sheep in mind!;-)