[hotfix][docs] Move PERCENTILE doc from arithmetic to aggregate section

[dylanhz]

What is the purpose of the change

The PERCENTILE aggregate function was incorrectly documented under the Arithmetic Functions section instead of the Aggregate Functions section in both the English and Chinese documentation YAML data files (sql_functions.yml / sql_functions_zh.yml).

This is a documentation-only fix — no code changes.

Brief change log

• Moved PERCENTILE entry from arithmetic to aggregate section in docs/data/sql_functions.yml
• Moved PERCENTILE entry from arithmetic to aggregate section in docs/data/sql_functions_zh.yml

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• be99140 Azure: SUCCESS

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• @flinkbot run azure re-run the last Azure build

[lincoln-lil]

LGTM！