Replace `m2r` with a package that is maintained

[JulioAPeraza]

Summary

Switch from m2r to m2r2 or sphinx-mdinclude.

Additional details

m2r was archived and thus no longer maintained. That also prevents us to upgrade mistune, which is used by other dependencies.

https://github.com/neurostuff/NiMARE/blob/4ceb3589820801d852ece67dbb1adacdbf77f7b3/setup.cfg#L64-L67

Next steps

Switch to m2r2 or sphinx-mdinclude. sphinx-mdinclude at least works with mistune 2.x.

[jdkent]

I do not have a strong opinion, m2r2 has more stars, but sphinx-mdinclude appears to be more focused as a sphinx-extension which is our use-case