Merge bitcoin#20929: doc: Move 0.21.0 release notes from wiki

66e6742 doc: Move 0.21.0 release notes from wiki (Wladimir J. van der Laan)

Pull request description:

  These were the contents of https://github.com/bitcoin-core/bitcoin-devwiki/wiki/0.21.0-Release-Notes-Draft, minus the paragraph about gauging how fast taproot can be deployed.

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Tree-SHA512: d7bb61a76d87cb9d9e43602bf81b51ea463e3a098748e42440950d08e6f6e84bf6c02b07b297ac959f979aa72682775dd2ae2be6d30b22977ed60ed454b9c66d