[wiki] Study Variables

[adamdaudrich]

Brief summary of changes

• This is a different approach to the docs; a way to more gradually introduce the complexity of the variables to the reader.
• The Pscid.md file is an example of how this approach could be structured.
• The CandID file is started.
• The Identifiers.md file is not needed, I feel.
• I was asked to correct a small typo (it's - > its) in the README.md
• I changed the 'Introduction to Study Variables' to become a concise branching document, linking to the other files

[CamilleBeau]

As per Dave, this should be rebased to main branch