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

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

https://ironfish.network/developers/documentation

https://www.rfc-editor.org/standards

On a more serious note:

https://www.rfc-editor.org/standards

On a more serious note: https://loops.so/docs/

Have you checked vocs.dev?
I like drizzle.team’s AI Search, quite useful.
Are you looking for UI or UX inspiration?

Stripe docs best docs hands down fight me

I almost forgot to mention "https://ethroadmap.com/" as the best INTERACTIVE/Modern informative website.

* Make sure to check it out if you haven't yet.

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...