Add Datadog integration doc for TiDB Cloud Premium and Essential (Preview)

[huoyao1125]

What is changed, added or deleted? (Required)

Add a new documentation page tidb-cloud/monitor-datadog-integration-for-tidb-x.md for the Datadog integration feature for TiDB Cloud Premium and Essential (Preview).

The doc covers:

• Prerequisites and limitations
• Step 1: Configure the Datadog API key integration from the TiDB Cloud console
• Step 2: Manually download and import the pre-built Datadog dashboard JSON (because the TiDB Cloud dashboard for Premium/Essential is not yet available in the Datadog marketplace)
• How to view the pre-built dashboard
• Full metrics reference table for both Essential and Premium tiers

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)

What is the related PR or file link(s)?

• Reference: tidb-cloud/monitor-datadog-integration.md (Dedicated Datadog integration doc)
• Reference: tidb-cloud/prometheus-grafana-integration.md (Premium/Essential Prometheus integration doc)
• Dashboard JSON (Essential): https://github.com/tidbcloud/metrics-integration/blob/main/artifacts/dashboard/datadog/essential-datadog.json
• Dashboard JSON (Premium): https://github.com/tidbcloud/metrics-integration/blob/main/artifacts/dashboard/datadog/premium-datadog.json

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Code Review

This pull request creates a new, empty markdown file named monitor-datadog-integration-for-tidb-x.md. The reviewer pointed out that the file is empty and requested that the documentation content be added or the file be removed if it was created by mistake.

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[qiancai]

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ref: https://github.com/pingcap/docs/blob/b03a361a02552d33bcb22798f7856fe51b8ceeda/tidb-cloud/monitor-prometheus-and-grafana-integration-grafana-dashboard-UI.json