docs: Add PHP OpenAI compatibility & doc

[PROFeNoM]

What does this PR do? What is the motivation?

The implementation of the OpenAI Integration was just merged in the PHP Tracing Library: https://github.com/DataDog/dd-trace-php/pull/2685

This updates the corresponding public doc.

Merge instructions

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/alex/docs/openai-php/tracing/trace_collection/compatibility/php
• https://docs-staging.datadoghq.com/alex/docs/openai-php/tracing/trace_collection/library_config/php