docs: add HTTP API for dynamic trace control

[Copilot]

What's Changed in this PR

Documents the /debug/enable_trace HTTP endpoint added in GreptimeTeam/greptimedb#6609, enabling runtime control of distributed tracing without restart.

Changes:

• docs/reference/http-endpoints.md: Added endpoint documentation under Admin APIs with POST examples
• docs/user-guide/deployments-administration/monitoring/tracing.md: Added "Dynamic Tracing Control" section with usage patterns

Usage:

# Enable tracing
curl --data "true" http://127.0.0.1:4000/debug/enable_trace

# Disable tracing  
curl --data "false" http://127.0.0.1:4000/debug/enable_trace

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update docs for feat: dynamic enable or disable trace</issue_title>
<issue_description>A document change request is generated from GreptimeTeam/greptimedb#6609</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update docs for feat: dynamic enable or disable trace #2000

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[fengjiachun]

The Chinese document also needs updating, right?