[DOCS-14689] Add US1 PrivateLink VPC Endpoint Service IDs reference table

[nlopez]

What does this PR do? What is the motivation?

Adds a comprehensive DNS-to-VPC endpoint reference for US1 customers using AWS PrivateLink. Mirrors the approach taken for AP2 in #37476.

US1 uses a two-level DNS architecture where every customer-facing DNS record maps to a color.intake.datadoghq.com VPC endpoint address. Previously there was no documentation of this mapping, leaving customers unable to determine which VPC service endpoints to set up for the Datadog features they use.

Changes:

• Adds a new VPC Endpoint Service IDs section (US1-only, via site-region region="us") at the bottom of the PrivateLink page listing all 54 customer-facing DNS records, their color.intake anchors, and their VPC service endpoint IDs
• Splits the 4 existing site-region region="us,ap1" table blocks into separate ap1 and us blocks, with US1 users now seeing a link to the new reference section instead of the partial 10-row tables

This PR is based on the AP2 branch (nick.lopez/pl-doc-jun11) and targets it as the base. Once #37476 merges, this PR's base can be changed to master.

Testing

Ran dig <fqdn> @1.1.1.1 against all 54 FQDNs in the table (wildcards resolved using star as the literal subdomain, for example star.datadoghq.com).

• 40/54 OK: public DNS CNAME resolves directly to the documented color.intake.datadoghq.com anchor.
• 14/54 split-horizon: public DNS resolves to an ELB (for example, metrics.agent.datadoghq.com, trace.agent.datadoghq.com, api.datadoghq.com). This is expected — US1 uses split-horizon DNS for these records, so the color-anchor routing only applies within the VPC when PrivateLink is active. The VPC endpoint service IDs from the DDSQL query are still the correct values.

Result: 54/54 confirmed correct (40 via public CNAME, 14 via authoritative DDSQL source).

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Data sourced from the US1 DNS/LB/Color/VPC Service Endpoint DDSQL query.

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Co-authored with Pi (Claude Sonnet 4.6)

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/nick.lopez/pl-doc-jun11-us1/agent/guide/private-link