Add Observability (telemetry export) guide

[tkosushi]

https://github.com/tailor-inc/platform-core-services/issues/9829

Summary

User-facing documentation for the telemetryrouter feature: forwarding a workspace's OpenTelemetry traces and logs to external observability backends via the Tailor Terraform provider.

Guide: docs/guides/opentelemetry.md (/guides/opentelemetry, titled Observability / OpenTelemetry).

The guide is written around the platform user's mental model — the user configures telemetry exports, while TelemetryRouter is the internal component that routes a workspace's signals (and the source of the tailor_telemetryrouter_ resource prefix).

• Intro / How telemetry export works — distinguishes external export from in-console log viewing; concept diagram (signals → TelemetryRouter → your telemetry exports); per-export signal toggles; Secret Manager–based auth
• Configuring a telemetry export — tailor_telemetryrouter_telemetry_export, api_key / bearer_token auth. The full argument reference points to the Terraform Registry rather than duplicating tables
• Endpoint format — HTTP/gRPC formats, automatic signal-path append, TLS guidance, private-host rejection
• Backend examples — leads with direct-OTLP backends (New Relic, Honeycomb); the OpenTelemetry Collector path (Datadog) is presented as one option
• Resource attributes — tailor_telemetryrouter_resource_attributes_config (service name prefix, deployment environment)

Accuracy notes (verified against the telemetryrouter implementation)

• Credentials: the doc recommends the auth block (Secret Manager–backed). headers is marked sensitive but its values persist in Terraform state, so it is documented for non-secret headers only.
• Datadog: Datadog offers a direct OTLP intake endpoint (traces in preview, with constraints), so the collector is described as the full-featured path rather than a hard requirement.
• TLS: wording matches the implementation — only private/internal hosts are rejected at config time; there is no http:// rejection. HTTPS is recommended.
• Metrics: enable_metrics exists, but the platform does not emit metric signals today, so the guide scopes destinations to traces and logs.

Also adds the guide to docs/guides/index.md, fixes the guides nav order entry in .vitepress/config/constants.ts (observability → opentelemetry), and allowlists docs.datadoghq.com in schema.yml.

Verification

• pnpm build succeeds (internal links, anchors, mermaid, markdown all compile); page generated at /guides/opentelemetry
• schema-check / link-check / typo_check run in CI