docs: gov client spec (cosmos#10196)

## Description

Ref: cosmos#9707

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

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### Author Checklist

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please add links to any relevant follow up issues.*

I have...

- [x] included the correct `docs:` prefix in the PR title
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [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)
- [x] reviewed "Files changed" and left comments if necessary
- [x] confirmed all CI checks have passed

### Reviewers Checklist

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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)