`recommonmark` is deprecated

[sobolevn]

https://github.com/readthedocs/recommonmark#recommonmark

We should switch to myst: https://github.com/executablebooks/MyST-Parser

[Saransh-cpp]

Hi @sobolevn, I was looking through the codebase and I couldn't find any usage of recommonmark? It is available in the requirements.txt file but isn't available in the conf.py file (which it should be, according to the documentation). Could you please let me know about its usage in the code?

[geekusa33]

@sobolevn I updated the requirements.txt file to deprecate recommonmark and add myst-parser. It is on PR #315

completed as part of hacktoberfest 2021