[CCR-3457] Document cost recommendations mcp tool

[eliottmc]

What does this PR do? What is the motivation?

Documents the new cost_recommendations tool in the Datadog MCP Server, which lets an AI agent query Cloud Cost Management cost-saving recommendations.

Changes:

• mcp_server/tools.md: Adds a Cloud Cost Management section documenting the cost_recommendations tool, including its toolset, required permissions, description, and example queries.
• mcp_server/setup.md: Adds the cost toolset to the list of available toolsets.
• cloud_cost_management/recommendations/_index.md: Adds a note pointing readers to the cost_recommendations MCP tool for querying recommendations from an AI agent.
• cloud_cost_management/cloud_cost_skill.md: Expands the "Use the Datadog MCP Server for cost analysis" section to explain which toolset to enable for each task. Adds a "List cost-saving recommendations" subsection covering the cost toolset and cost_recommendations tool (with required permission and example queries), and groups the existing metric tools under a "Query Cloud Cost Management metrics" subsection for the core toolset.

Merge readiness

• Ready for merge

Associated Jira ticket: https://datadoghq.atlassian.net/browse/CCR-3457

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/eliott.mace/ccr-3457-cost-recommendations-mcp-docs/cloud_cost_management/cloud_cost_skill
• https://docs-staging.datadoghq.com/eliott.mace/ccr-3457-cost-recommendations-mcp-docs/cloud_cost_management/recommendations/
• https://docs-staging.datadoghq.com/eliott.mace/ccr-3457-cost-recommendations-mcp-docs/mcp_server/setup
• https://docs-staging.datadoghq.com/eliott.mace/ccr-3457-cost-recommendations-mcp-docs/mcp_server/tools

[github-actions]

⚠️ Possible site support note detected

The following files have a site-region shortcode at the top of the page that mentions a feature being "not available" or "not supported":

• content/en/mcp_server/setup.md

Are you trying to document site support for a product? If so, the site-region shortcode is not the recommended approach. See the Documenting site support guide for the preferred method.