docs(budget): example for creating budgets for Genie usage

[zcking]

Changes

Update docs with an example illustrating how to create and manage Budgets for Genie now that Genie is going PAYGO next month.

UI changes make this feel like a totally different set of fields ("resource tags") but it's actually the same old familiar filter { ... tags { ... } } configuration. The example makes this more obvious for users looking to manage this specific cost control.

Tests

• relevant change in docs/ folder
• make fmt-docs run
• renderes successfully in Terraform Registry Doc Preview Tool
• tested the example HCL configuration in internal account

[alexott]

lgtm, only either add changelog item, or add a corresponding tag into the PR description

[github-actions]

Unit tests

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Inputs:

• PR number: 5817
• Commit SHA: b2d826c1da337f60ce312d3aca1052e9e7b58bf0

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