[DOC] Fix links and sizes in approach documentation

[tsalo]

Closes None. I noticed that the Processing pipeline details page had some mistakes with the figures ever since I updated the notebook that generates them. This should fix that.

Changes proposed in this pull request:

• Delete PCA figure that says "whiten" instead of "dimensionally reduce".
• Re-generate figures with improved dimensions.
• Adjust rendering dimensions in approach.rst.
• Fix use of cumsum in MLE PCA for variance explained. Luckily this doesn't impact anything.

[codecov]

Codecov Report

Merging #369 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #369   +/-   ##
=======================================
  Coverage   47.41%   47.41%           
=======================================
  Files          39       39           
  Lines        2223     2223           
=======================================
  Hits         1054     1054           
  Misses       1169     1169

Impacted Files	Coverage Δ
tedana/decomposition/pca.py	14.77% <0%> (ø)	⬆️

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[emdupre]

🎉 Thanks !