Add developer docs on Penumbra SDK

[conorsch]

The current dev docs https://guide.penumbra.zone/dev don't sufficiently explain how to code against Penumbra, when writing integrations or applications. We should absolutely call out the fact that although Penumbra is a Cosmos chain, one cannot use the Cosmos SDK. This violation of assumptions has caused friction for several devs, so let's write it down. Consider the following feedback shared from a recent developer:

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• Penumbra documentation suggestions
  • RPC service
    • Create a document that outlines how to interact with the tendermint-compatible blockchain service using a unique transaction format based on actions instead of traditional messages.
    • Describe supported endpoints , like status, blocks, block_results, tx, and events subscriptions.
• Transaction payload
  • Transactions are structured around actions rather than messages. Explain the payload differences with examples.
• Actions
  • Describe supported actions like spend,output, undelegateA
  • Describe tx parameters, detection data, etc
• Events
  • List supported events, and show how clients can subscribe to various events emitted by the blockchain.
  • Describe supported events e.g. EventBlockRoot, EventAnchor , etc

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In a nutshell, we need to highlight the importance of using custom protobuf messages via the buf tooling. That would go a long way in saving new devs time while orienting themselves to build against Penumbra.