Add links to release notes

[beckykd]

closes #1538

How do we want these links to appear? Here are some different ways I could add them, but all of these options seem a little heavy-handed and will be hard to properly maintain:

• In the intro: https://qiskit.github.io/documentation/pr-1782/guides/circuit-library
• At the very top in italics: https://qiskit.github.io/documentation/pr-1782/guides/construct-circuits
• At the bottom in Next steps (also note that I have the link in the intro section, where I think that people would miss it): https://qiskit.github.io/documentation/pr-1782/guides/classical-feedforward-and-control-flow
• As part of a "what's required" section https://qiskit.github.io/documentation/pr-1782/guides/synthesize-unitary-operators

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[beckykd]

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