Add `--template` option to pyscript create

[jdw170000]

Pursuant to issue #7, this PR adds a --template command line argument which can be passed to the pyscript project creation script.

The --template option allows users to name a custom template that they have created in the templates folder to use instead of the default basic.html template.

The PR also includes a new unit test to validate the --template option, as well as a description of the option in the documentation.

Written at PyCon US 2024 sprints.