Add directive to document workflows

@denolf suggested that a directive could auto-generate workflow documentation like this one: https://ewoksfluo.readthedocs.io/en/latest/howtoguides/xrfmap_single_detector.html

The directive could:

• Go through the workflow JSON to find the input parameters (task inputs not covered by a link) and list them
• Generate a visual representation of the JSON (e.g. as a mermaid diagram)