Example: add API catalog style with multiple sidebars

[adamaltman]

This PR links the 3 thin tiles on the home page of the portal to different APIs. Each API has been given its own sidebar. On the other hand, it would be possible to put them all on the same sidebar:

all-apis:
  - page: openapi/demo.page.yaml
  - page: openapi/demo-parameters.page.yaml
# etc...

This demonstrates different styles.

Notice one has "expand: false" in the settings -- this means it will have a single page style.

Notice in the sidebar, one has the /*. This shifts the APIs inside of the definition one group in the navigation.