website: Add wandering Plugin Signing page to publishing sidebar
website: Add wandering Plugin Signing page to publishing sidebar

This page file was already marked as belonging to the registry publishing docs,
but was unreachable because it wasn't included in the nav.

rnaveiras/terraform,ewbankkit/terraform,hashicorp/terraform,ewbankkit/terraform,tommynsong/terraform,hashicorp/terraform,pdecat/terraform,pdecat/terraform,apparentlymart/terraform,apparentlymart/terraform,rnaveiras/terraform,pdecat/terraform,ewbankkit/terraform,rnaveiras/terraform,apparentlymart/terraform,tommynsong/terraform,ewbankkit/terraform,pdecat/terraform,apparentlymart/terraform,apparentlymart/terraform,tommynsong/terraform,hashicorp/terraform