Write documentation for how to write a treatment file

[JamesPHoughton]

Should include:

• a description of the major sections (introSequences, treatments) and in each of the gamestages,
• a section for each of the allowed components which are generally described here: https://github.com/Watts-Lab/deliberation-empirica/tree/main/client/src/elements, you may also want to look at the validator code
• allowed markdown syntax (can be links to other documentation, just making sure that we point to the right stuff)
• a description of the markdown frontmatter syntax for prompts, and the different sections for prompts and question lists