Add get_root_span_breakdown MCP tool documentation

[jschoijet]

• Add tool to Available tools table
• Add "Investigate span performance" use case with example queries

🤖 Generated with Claude Code

Description of changes

[eg., "This PR clarifies the WebSDK docs, explaining how the Severity Score is calculated"]

Checklist before you pull:

• Double-check that any changes fit the Embrace Docs Style Guide.
• Please ensure that there is no customer-identifying information in text or images.
• If you changed any doc file names, add a redirect from old to new URL so that we don't end up with broken links. You can do this by adding a set of to/from values in docusaurus.config.js under @docusaurus/plugin-client-redirects.
• If you link to a header within a page in the docs, make sure that header has an explicit tag in case it changes in the future. E.g., the H3 header ### Hello World {#my-explicit-id} has an explicit tag my-explicit-id.

[github-actions]

Summary - Markdown Lint

Success: No issues found in 392 files (99ms)

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Summary - Link Checker

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