Fill in "Getting Started with CWL" page

[jaeddy]

(this one: https://github.com/Sage-Bionetworks/sage-workflows-sandbox/blob/master/docs/cwl_usage/cwl_getting-started.rst)

Some ideas on topics to include from notes in Synapse (Dockstore docs might be good source of inspiration for some of these?):

• CWL developer toolkit: environment setup, tools, and tips
• installing cwltool
• Writing your first workflow (e.g., "hello world")
• Running a toy job on your laptop, and how to quickly get a workflow running in the cloud