Allow a named component to be promoted to the site root

There are two goals we want to accomplish:

• Provide a start page / entry point for the site
• Hide the component folder for a component, often when the site only has a single component

Add configuration to the playbook that allows a component to be promoted to the root of the site. This simply means that the path segment for the component is dropped from the URL. In other words, /my-component/index.html becomes /index.html. We'll call this the site component.

NOTE: None of the source-to-source references to this component change. This only affects how the publication URL is computed.

The proposed configuration is as follows:

site:
  component: my-component

The only caveat is that, in order for the site component to occupy the root of the site, it must be versionless (or have a versionless entry). Otherwise, /my-component/1.0/index.html just becomes /1.0/index.html. To solve this problem, we may also allow the version to be specified:

site:
  component: 1.0@my-component

In this case, /my-component/1.0/index.html would become /index.html. So it's really promoting a single component version. But what about the other versions of that component? Do they get promoted at all?

Another way to think about it is to have a start page for the site, which defaults to the start page for the greatest version of the site component. In other words:

site:
  component: my-component
  # if not defined here, use the start page for the greatest version of site component
  start_page: 1.0@my-component::index.adoc

So when the visitor navigates to the docs domain, the visitor is redirected automatically to the start page (/index.html -> /1.0/index.html).