Introduce how to attach database migration scripts for custom plugins in Kubernetes deploymnet

[randmonkey]

Where is the problem?

https://docs.konghq.com/kubernetes-ingress-controller/latest/plugins/custom/#deploying-custom-plugins

What happened?

Custom plugins with database migration scripts needs a migrations sub-directory in the plugin's directory including the migration scripts. But ConfigMap volumes cannot support nested directories, so users cannot attach the migration scripts following the steps of deploying custom plugins. The document need a section to introduce how to attach the migration scripts into the container separately.

What did you expect to happen?

The document contains the steps of deploying database migration scripts of custom plugins.

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