RFC: Add a REPL manual section: "Interacting with Julia"

Here's a start at documenting the REPL.  There's some overlap with the "Getting Started with Julia" section at the beginning that seemed necessary to establish context.

This can be merged as-is, but a few questions:

Is this where it should go?  Or should be combined with getting started with Julia?  Should it be its own page?  I kept it as its own page as I could see a section on IJulia here, too.

Fixes #4842.