Document .EdgeQL CLI tool

[i0bs]

Related to #3, documentation is preferred for users wanting to find out about the CLI through the EdgeDB website. We can most likely use the :cli:synposis: Sphinx callable for all of the documentation.

This means we can also add admonition notes in areas where we want to inform users that there's a way they could perform an action through the CLI instead, such as automatically generating classes for query execution.