Add choose content topic guide

guides/choose-content-topic.md

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+# How to choose a content topic
+
+A content topic is used for content based filtering.
+
+It allows you to filter out the messages that your dApp processes,
+both when receiving live messages (Relay) or retrieving historical messages (Store).
+
+The format for content topics is as follows:
+
+`/{dapp-name}/{version}/{content-topic-name}/{encoding}`
+
+- `dapp-name`: The name of your dApp, it must be unique to avoid conflict with other dApps.
+- `version`: We usually start at `1`, useful when introducing breaking changes in your messages.
+- `content-topic-name`: The actual content topic name to use for filtering.
+  If your dApp uses DappConnect for several features,
+  you should use a content topic per feature.
+- `encoding`: The encoding format of the message, we recommend using Protobuf: `proto`.
+
+For example: Your dApp's name is SuperCrypto,
+it enables users to receive notifications and send private messages.
+You may want to use the following content topics:
+
+- `/supercrypto/1/notification/proto`
+- `/supercrypto/1/private-message/proto`
+
+You can learn more about Waku topics in the [23/WAKU2-TOPICS](https://rfc.vac.dev/spec/23/) specs.