Replace `m2r` with a package that is maintained

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m2r was archived and thus no longer maintained.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.

NiMARE/setup.cfg

Lines 64 to 67 in 4ceb358

    
           doc = 
        
               m2r 
        
               matplotlib 
        
               mistune<2  # just temporary until m2r addresses this issue

Next steps

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

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

	doc =
	m2r
	matplotlib
	mistune<2 # just temporary until m2r addresses this issue