Add list_root_spans MCP tool documentation

[santileira]

Description of changes

Add documentation for the new list_root_spans MCP tool:

• Add tool to the Available tools table
• Add "Identify slow or failing operations" use case with example queries showing different order_by and filter options

This documents the backend work from embrace-io/go#9387.

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 (98ms)

[github-actions]

Summary - Link Checker

Status	Count
🔍 Total	471
✅ Successful	377
⏳ Timeouts	0
🔀 Redirected	0
👻 Excluded	94
❓ Unknown	0
🚫 Errors	0
⛔ Unsupported	0
Full Github Actions output