Add docs badge

[tobiassjosten]

What does this PR do?

This adds a badge to the README, linked to the package documentation on pkg.go.dev.

Additional Notes

Nothing to add.

Review checklist

Please check relevant items below:

• [X] This PR includes all newly recorded cassettes for any modified tests.

• [X] This PR does not rely on API client schema changes.

  • [X] The CI should be fully passing.

• [ ] Or, this PR relies on API schema changes and this is a Draft PR to include tests for that new functionality.

  • Note: CI shouldn't be run on this Draft PR, as its expected to fail without the corresponding schema changes.

[github-actions[bot]]

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