docs: gov client spec (#10196)

## Description

Ref: #9707

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Add `client.md` page to `gov` module spec.

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### Author Checklist

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- [x] included the correct `docs:` prefix in the PR title
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- [x] provided a link to the relevant issue or specification
- [x] followed the [documentation writing guidelines](https://github.com/cosmos/cosmos-sdk/blob/master/docs/DOC_WRITING_GUIDELINES.md)
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I have...

- [ ] confirmed the correct `docs:` prefix in the PR title
- [ ] confirmed all author checklist items have been addressed 
- [ ] confirmed that this PR only changes documentation
- [ ] reviewed content for consistency
- [ ] reviewed content for thoroughness
- [ ] reviewed content for spelling and grammar
- [ ] tested instructions (if applicable)

x/gov/spec/README.md

@@ -55,3 +55,7 @@ staking token of the chain.
     - [Handlers](04_events.md#handlers)
 5. **[Future Improvements](05_future_improvements.md)**
 6. **[Parameters](06_params.md)**
+7. **[Client](07_client.md)**
+    - [CLI](07_client.md#cli)
+    - [gRPC](07_client.md#grpc)
+    - [REST](07_client.md#rest)