Additional blog posts about package adoption

[ericholscher]

We should definitely do a "most popular Sphinx extensions" post, using our data & PyPI stats if we can find them. Also there's an interesting one around Sphinx version/theme adoption using our data & https://pepy.tech/project/sphinx & https://pepy.tech/project/sphinx_rtd_theme (edited)

[humitos]

We should definitely do a "most popular Sphinx extensions" post

Before writing this blog post, I'd like to have this PR merged and collect data during some weeks first: readthedocs/readthedocs.org#9639. It will allow us to detect more Sphinx extensions that do not start with sphinxcontrib- or sphinx- on their package name.

It will also give us more realistic data about what's the theme the documentation project is using.

[humitos]

This is the previous post we wrote and the one we should use as reference to write an updated version of it: https://blog.readthedocs.com/knowing-more-about-ourselves/