The GDS markdown guide is available online.
https://www.gov.uk/guidance/how-to-publish-on-gov-uk/markdown
However, it's advice on some things is WRONG. Here are the right instructions
Sub or superscripts
For subscript, surround the text to change with the <sub> </sub> markdown.
This is an example of using subscript in text.
For superscript, surround the text to change with the <sup> </sup> markdown.
This is an example of using superscript in text.
Linking to footnotes/references
Put the reference numbers in square brackets next to the text in the page, for example:
The second edition was subsequently published in 2010 [^1].
Then in the references at the foot of the document, mark them thus:
[^1]: Burford D, Kirby M, Austoker J. Prostate Cancer Risk Management Programme information for primary care; PSA testing in asymptomatic men. [Evidence document. NHS Cancer Screening Programmes; 2010 Jan](http://www.cancerscreening.nhs.uk/prostate/pcrmp-guide-2.html). Accessed June 2014
Tables
If you need to add line breaks in a table cell, use the </br> tag.
If you need to convert large amounts of text into a table, use the table convertor: