Add help text to the editor

[cajosc]

• Link to code book and guidelines from the editor #72
• Make it possible to toggle display of the full explanation of taxonomy #20 (postponed as of Make it possible to toggle display of the full explanation of taxonomy #20 (comment))

[arildm]

Pull raw Markdown files directly and render in-line.

[arildm]

Pull raw Markdown files directly and render in-line.

That may be possible with some kind of auth token, not sure. The current solution instead grabs the HTML from the GitHub pages, e.g. https://spraakbanken.github.io/swell-project/Anonymization_guidelines

[elenavolodina]

Looks good to me