feat(docs): Update README.md to add link to recently-added GCP integration document.

[michaelsafyan]

Description

Improve discoverability of the GCP integration document by linking to it from the OpenLLMetry README.

Task List

• [ N/A] I have added tests that cover my changes.
• [N/A ] If adding a new instrumentation or changing an existing one, I've added screenshots from some observability platform showing the change.
• [ X] PR name follows conventional commits format: feat(instrumentation): ... or fix(instrumentation): ....
• [ X] (If applicable) I have updated the documentation accordingly.

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[nirga]

Thanks @michaelsafyan!