Switch to MyST-Parser from recommonmark for parse_markdown function

[jpmckinney]

Related: #24

[jpmckinney]

recommonmark will be sunset in about a month: readthedocs/recommonmark#221

[Bjwebb]

Thanks for the heads up, I'm expecting we'll move all the sphinx sites we're responsible for over to MyST, although it may not happen until we come to update the requirements anyway.

[jpmckinney]

Thanks for the heads up, I'm expecting we'll move all the sphinx sites we're responsible for over to MyST, although it may not happen until we come to update the requirements anyway.

When you do, you can:

• Eliminate the .. markdown directive, since it was only useful to fix a bug in recommonmark, where the content of a directive would be parsed as RST. (MyST parses everything as Markdown if the filename ends with .md)
• Eliminate use of the parse_markdown function. As above, this is only needed when using recommonmark. The effect when using MyST is of parsing the same content twice.