[CCT-1506] Feature documentation for custom forecast

[dmytro-kliagin-dd]

What does this PR do? What is the motivation?

Documents the custom forecast feature for Cloud Cost Management budgets:

• Budgets page: Adds a "Customize your budget forecast" section covering the Bits forecast vs. custom forecast vs. final forecast concepts, how to add or edit custom forecast values when creating or editing a budget, the forecast cell states, and how custom forecasts are used in budget status, monitors, CSV export, and on delete.
• Forecasting page: Adds a "Bits and custom forecasts" concept section and a cross-link from "View forecasts in budgets" to the Budgets how-to.

Scope is limited to the V1 release. Fast-follow items (view toggle between Bits and final forecast, budget reports integration, Terraform, CSV bulk upload, edit history) are intentionally not documented.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Holding merge until the feature is generally available.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/dmytro.kliagin/CCT1506-custom-forecast-docs/cloud_cost_management/planning/budgets
• https://docs-staging.datadoghq.com/dmytro.kliagin/CCT1506-custom-forecast-docs/cloud_cost_management/planning/forecasting

[drichards-87]

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