Update README.md (#4502)

..and other high level docs.


# Polling 

Please vote in the reactions of this PR 

- 👍 I agree to replace the website of this repo to
https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_docs/index.html
- 👎 Keep it as polkadot.network
- 🤷

 Different opinion

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Co-authored-by: Bastian Köcher <[email protected]>