Allow precision-recall to handle 2d predicted probabilities; docs updates

[dsherry]

Changes:

• Minor fix to precision-recall graph method: use same code as ROC method to process input data
• User guide updates: train_pipelines/score_pipelines, automl and pipeline save/load (fix Add user guide entry for pipeline save/load methods #2083 ), move DT viz lower in model understanding guide. Update automl guide to use training-holdout split.
• Add note to contributers guide about GitHub Actions permissions
• Fix typo in PipelineBase error.

[codecov]

Codecov Report

Merging #2090 (d6dc364) into main (1cb0724) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #2090     +/-   ##
=========================================
+ Coverage   100.0%   100.0%   +0.1%     
=========================================
  Files         296      296             
  Lines       24677    24712     +35     
=========================================
+ Hits        24659    24694     +35     
  Misses         18       18             

Impacted Files	Coverage Δ
evalml/exceptions/__init__.py	100.0% <ø> (ø)
evalml/pipelines/pipeline_base.py	99.7% <ø> (ø)
evalml/exceptions/exceptions.py	100.0% <100.0%> (ø)
evalml/model_understanding/graphs.py	100.0% <100.0%> (ø)
...lml/tests/model_understanding_tests/test_graphs.py	100.0% <100.0%> (ø)

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[chukarsten]

An approval for this PR, I have given. Ignore the grammar comments, you may.

[chukarsten]

Begun, the Grammar Wars have.