Document OverrideExperimentByGroupName for supported Server Core SDKs

[trentusus]

Summary

Adds OverrideExperimentByGroupName documentation where it was missing in the Server Core docs, while preserving the existing snippet templates and examples for SDKs that already had local overrides docs.

Changes

• add PHP Local Overrides docs and surface them on the PHP Server Core page
• add overrideExperimentByGroupName / override_experiment_by_group_name to the Node and Python reference sections
• add group-name experiment override examples to the Java/Kotlin local overrides snippet
• add a group-name experiment override example to the Rust local overrides snippet

Notes

• kept existing Node, Python, and .NET local override examples in their current template/style
• did not broaden this into a docs normalization pass
• left unsupported SDK pages unchanged

Validation

• verified the local Mintlify preview served the touched Server Core pages successfully
• opened the Node, Python, Java, .NET, Rust, and PHP Server Core pages against the local preview
• manually reviewed the rendered docs pages to confirm the new examples and references were correct
• known repo-wide Mintlify baseline warnings remain separate from these page-local changes