Add documentation for velog and placeholder in the new docsite
Add documentation for velog and placeholder in the new docsite

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171080133

google/closure-templates,google/closure-templates,yext/closure-templates,Medium/closure-templates,Medium/closure-templates,Medium/closure-templates,yext/closure-templates,yext/closure-templates,yext/closure-templates,Medium/closure-templates,google/closure-templates,google/closure-templates,Medium/closure-templates,google/closure-templates