Add blog post about Sphinx and Markdown

[astrojuanlu]

I have experimented with different approaches, and finally settled with this. Talking only about our conferences seemed a weird thing to do, so instead I framed them as part of our larger efforts to promote Markdown, which are also in line with the CZI grant. Let me know what you think @ericholscher

[astrojuanlu]

🤖 Rendered version: https://readthedocs-blog--147.org.readthedocs.build/sphinx-markdown-2021/