Document browser profiling quota API endpoint and proxy/CSP setup

[thomasbertet]

What does this PR do?

Adds documentation for the quota check API call made by the Browser SDK when Browser Profiling is enabled.

Changes

content/en/real_user_monitoring/_index.md

• Adds a new "Additional endpoints for Browser Profiling" subsection under "Supported endpoints for SDK domains", listing the quota. subdomain URLs for all 8 Datadog sites (e.g. https://quota.browser-intake-datadoghq.com).
• Includes a note that proxy and CSP users must allow these domains.

content/en/real_user_monitoring/correlate_with_other_telemetry/profiling/_index.mdoc.md

• Adds a new Step 3 "Quota check" in the browser setup section, explaining:
  • The SDK calls /api/v2/profiling/quota before starting a profiling session
  • The call goes to a quota. subdomain of the standard RUM intake
  • Proxy and CSP users must explicitly allow this domain

content/en/real_user_monitoring/guide/proxy-rum-data.mdoc.md

• Documents the ddforwardSubdomain query parameter sent when using a string proxy, which tells the proxy which intake subdomain to target (quota for profiling quota requests).
• Documents the subdomain property in the function proxy options object (alongside existing path and parameters), used for the same purpose.

Motivation

The Browser SDK's profiling quota check reaches a new quota. subdomain that wasn't previously documented. Customers using a proxy or CSP restrictions would silently fail to collect profiling data without this information.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/thomas.bertet/browser-profiling-quota-docs/real_user_monitoring/
• https://docs-staging.datadoghq.com/thomas.bertet/browser-profiling-quota-docs/real_user_monitoring/correlate_with_other_telemetry/profiling/
• https://docs-staging.datadoghq.com/thomas.bertet/browser-profiling-quota-docs/real_user_monitoring/guide/proxy-rum-data