Document Support to register scaffolderPlugin.provide(createScaffolderFieldExtension) for dynamic plugin

Release notes text: This change adds a new "scaffolderFieldExtensions" configuration option to a dynamic plugin's frontend configuration. This option allows a dynamic plugin to specify one or more exported components that will be provided to the scaffolder plugin as field extensions. Scaffolder field extensions provide custom form field components to the software template wizard.

Upstream PR: https://github.com/janus-idp/backstage-showcase/pull/1253