Split data dictionary into separate markdown tables by SQL table (fix…

[dmolesUC]

If this pull request addresses an open issue on the repository, please add 'Closes #NN' below, where NN is the issue number.

Closes #384.

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

• Separates the data dictionary into three separate Markdown tables (one per SQL table), so as not to confuse students when they first import only the surveys table
• Rearranges fields into the order they appear in the DB Browser for SQL Lite interface (= the order of columns in the CSV, which also becomes the SQL declaration order), so it's easier for students to read down the table as they set data types

If any relevant discussions have taken place elsewhere, please provide links to these.

See #384.

[github-actions]

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