Multipage app - Automatically create default navigation

#### Is your feature request related to a problem? Please describe.
When using `panel serve` to serve multiple pages for a multipage app there is no navigation between pages.

#### Describe the solution you'd like
Panel automatically creates navigation links between pages. Something similar to what Streamlit does would be nice (see https://docs.streamlit.io/library/get-started/multipage-apps/create-a-multipage-app).

#### Describe alternatives you've considered
1. Other forms of navigation would be good.
2. Be able to provide a super simple file to specify the layout that gives the files to display in the navigation or a simple template that can be used.

#### Additional context
For when you have multiple apps, each in a separate file but would like to use them together or to be able to easily switch between them. Like having a home page for apps that tend to be used at the same time.