[DOCS-14568] GA NGINX Single-Step Instrumentation

[rtrieu]

What does this PR do? What is the motivation?

Fixes DOCS-14568

NGINX RUM auto-instrumentation is now GA. Users can now pick from three clearly defined setup paths instead of a flat page with an unclear alternative method buried at the bottom.

• Removed the Preview label
• Restructured the page into three tabs so users can quickly find the right setup path for their situation:
  • Single-Step Instrumentation: run one command, everything is set up automatically
  • Managed Instrumentation: configure your RUM app in the UI first, then run the install command
  • Manual Instructions: download the module and configure NGINX yourself
• Each tab opens with a plain "Use this method if..." sentence so users can make the right choice without reading all three
• Renumbered all link references to be consecutive
• Clarified the shared limitations shortcode to say "does not support all advanced RUM configurations" (applies to all server instrumentation pages)

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/rtrieu/docs-14568-nginx-ga/real_user_monitoring/application_monitoring/browser/setup/server/nginx

[github-actions]

⚠️ Possible site support note detected

The following files have a site-region shortcode at the top of the page that mentions a feature being "not available" or "not supported":

• content/en/real_user_monitoring/application_monitoring/browser/setup/server/nginx.md

Are you trying to document site support for a product? If so, the site-region shortcode is not the recommended approach. See the Documenting site support guide for the preferred method.