resolves #379 add alias for component start page at ROOT:index.adoc if nothing interferes

No docs yet. I thought of 3 cases to test in document-converter. Several tests in content-classifier are modified to expect the new startPageId attribute needed to make this work. I suppose it would be possible to check a generated site for the static redirect page, is this worthwhile?

I discovered that if you don't specify a start page in antora.yml, but some page does have an explicit :page-aliases: ROOT:index.adoc, the component start page does work (without any of these changes).

There's also a slightly crude but effective sample project at https://gitlab.com/djencks/simple-examples/tree/issue-379-start-page-auto-redirect under multiple-components/start-page-redirect