feat(docs/op-stack): Add link to mailing list

docs/op-stack/src/README.md

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 **The OP Stack is the standardized, shared, and open-source development stack that powers Optimism, maintained by the Optimism Collective.**
 
+::: tip
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+[Stay up to date on The Superchain and the OP Stack by subscribing to the Optimism newsletter](https://optimism.us6.list-manage.com/subscribe/post?u=9727fa8bec4011400e57cafcb&id=ca91042234&f_id=002a19e3f0).
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+:::
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 The OP Stack consists of the many different software components managed and maintained by the Optimism Collective that, together, form the backbone of Optimism. 
 The OP Stack is built as a public good for the Ethereum and Optimism ecosystems.
 

docs/op-stack/src/docs/releases/bedrock/explainer.md

@@ -8,6 +8,12 @@ meta:
 
 ![Bedrock](https://dev.optimism.io/content/images/size/w2000/2022/12/bedrock-BLUE.jpg)
 
+::: tip
+
+[Stay up to date on The Superchain and the OP Stack by subscribing to the Optimism newsletter](https://optimism.us6.list-manage.com/subscribe/post?u=9727fa8bec4011400e57cafcb&id=ca91042234&f_id=002a19e3f0).
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+:::
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 Bedrock is the name of the first ever official release of the OP Stack codebase, which is a set of free and open-source modular components that work together to power Optimism.
 
 - To understand what is in the Bedrock release, keep reading.

docs/op-stack/src/docs/understand/explainer.md

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   content: summary_large_image
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+::: tip
 
+[Stay up to date on The Superchain and the OP Stack by subscribing to the Optimism newsletter](https://optimism.us6.list-manage.com/subscribe/post?u=9727fa8bec4011400e57cafcb&id=ca91042234&f_id=002a19e3f0).
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+:::
 
 The next major scalability improvement to the OP Stack after [Bedrock](../releases/bedrock/) is to introduce the concept of *a Superchain*: a network of chains that share bridging, decentralized governance, upgrades, a communication layer and more—all built on the OP Stack.