all: add support for ephemeral resources

[SBGoods]

Adds support for ephemeral resource documentation:

• generate: adds documentation generation support for the templates/ephemeral-resources directory and templates/ephemeral-resources.md.tmpl file
• migrate: adds migration support for website/docs/ephemeral-resources and docs/ephemeral-resources subdirectories
• validate: adds ephemeral-resources to the list of valid directory names