[CCV-3803] Add Cloud Cost Management Forecasts documentation

[rtrieu]

What does this PR do? What is the motivation?

Re-adds content from #32277, which was reverted in #33263 because the feature wasn't ready. The feature is now ready.

• Add new forecasting.md page with overview, usage instructions, and data requirements
• Add forecast screenshots showing the feature in reports and budgets (list + detail views)
• Add Forecasting entry to the Planning nav menu
• Update planning/_index.md to include Forecasting in the Key Features list
• Update budgets.md to add a "View forecasts in budgets" section with a link to the new page
• Add "View forecasts in reports" section to forecasting.md

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Re-land of #32277.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/rtrieu/ccm-forecasts-reports/cloud_cost_management/planning/forecasting

Modified Files

• https://docs-staging.datadoghq.com/rtrieu/ccm-forecasts-reports/cloud_cost_management/planning/
• https://docs-staging.datadoghq.com/rtrieu/ccm-forecasts-reports/cloud_cost_management/planning/budgets