Make a blog index page

[brian-rose]

Currently https://projectpythia.org/blog gives a 404 error instead of a nice welcoming Blog home page.

A simple step toward #520 is to make a simple page here that just lists the TOC of the blog contents. That should be easy to do with the myst toc directive.

[brian-rose]

#542 introduced folder hierarchies by publication year for the blog. The blog index page should be organized the same way.

[brian-rose]

Hmm, https://projectpythia.org/blog is still 404 because the blog landing page is currently at https://projectpythia.org/posts/blog.

I will see about a quick fix for that.

[jukent]

Easy fix. Opening a PR now