Add pigletfly(Wang Bing) as a sig-docs-zh-reviewer

[pigletfly]

I'd like to provide my reviews for Chinese translation docs.

/cc @zacharysarah @markthink

[k8s-ci-robot]

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[tengqm]

@pigletfly You don't have to become a member of reviewers to provide your reviews. When you have many high-quality reviews in the community, you will receive an invitation to the group. Thanks ahead for your contributions.

[bradtopol]

@pigletfly Please submit this as a new PR and assign me and I will approve. Sorry for the miscommunication on this.

[tengqm]

@pigletfly @bradtopol was right, although we have documented criteria for this, there are exceptions sometimes.