Merge bitcoindevkit/bdk#2210: docs(rpc): add Emitter::next_block and Emitter::mempool...

30fa17955b16d6a5a435bea8e2b9728e9e0b3701 docs(rpc): add Emitter::next_block and Emitter::mempool examples (Kyle 🐆)

Pull request description:

  Adds `# Example` rustdoc to `Emitter::next_block` and `Emitter::mempool`, which were flagged as missing in the #2006 review.

  Originally this PR also removed `example_bitcoind_rpc_polling`, but #2006 has since merged and removed all example crates, so that part is dropped after rebase. Only the doc additions remain.

  Part of the #2006 follow-up to document the components previously covered by the `examples/` crates.

ACKs for top commit:
  ValuedMammal:
    ACK 30fa17955b16d6a5a435bea8e2b9728e9e0b3701

Tree-SHA512: 28ade24c33f9714f94282a4594024bff9ed304c461b477b7ba5e95de64fbc5951c23a6a93ee623cc764e1e31971aaccac3aaebe4f615637d5b2de6d56bc32b56