Fix binder links and change branch name to main in stable

[namurphy]

This is to change the branch name in stable in an attempt to make binder work. This is different than #1159.

One thing I'm wondering about is why there was an attempt to access master when launching a binder notebook from the stable version of the docs. So the next thing to do is...

• Verify what version of PlasmaPy should get launched in binder

[codecov]

Codecov Report

Merging #1161 (10889f4) into stable (39f1723) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           stable    #1161   +/-   ##
=======================================
  Coverage   96.91%   96.91%           
=======================================
  Files          70       70           
  Lines        6929     6929           
=======================================
  Hits         6715     6715           
  Misses        214      214           

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[namurphy]

Closing this since we're going to do a release soon, which will make this obsolete.