Pipeline user guide: include description of how to access the component instances and features

[angela97lin]

Closes #967. Updated docs here: https://evalml.alteryx.com/en/967_pipeline_user_guide/user_guide/pipelines.html

• describes component_graph
• describes .estimator accessor
• mentions assumption that pipelines always end with an estimator
• mentions pipeline.get_component(name) and pipeline.component_graph[idx]
• mentions input_feature_names attribute on pipelines

Also updates pipeline guide to use Imputer instead of Simple Imputer since that's the new default for AutoML.

Note: We can't add references to the API references for component attributes until #652 is resolved.

[codecov]

Codecov Report

Merging #1163 into main will increase coverage by 8.70%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1163      +/-   ##
==========================================
+ Coverage   91.22%   99.92%   +8.70%     
==========================================
  Files         196      196              
  Lines       11995    11995              
==========================================
+ Hits        10942    11986    +1044     
+ Misses       1053        9    -1044     

Impacted Files	Coverage Δ
evalml/automl/automl_search.py	99.58% <0.00%> (+0.41%)	⬆️
evalml/tests/component_tests/test_components.py	100.00% <0.00%> (+0.51%)	⬆️
...s/prediction_explanations_tests/test_algorithms.py	100.00% <0.00%> (+1.11%)	⬆️
evalml/tests/component_tests/test_utils.py	100.00% <0.00%> (+3.57%)	⬆️
evalml/tests/pipeline_tests/test_pipelines.py	100.00% <0.00%> (+4.17%)	⬆️
...derstanding/prediction_explanations/_algorithms.py	97.14% <0.00%> (+4.28%)	⬆️
evalml/utils/gen_utils.py	99.02% <0.00%> (+5.82%)	⬆️
evalml/tests/utils_tests/test_dependencies.py	100.00% <0.00%> (+6.25%)	⬆️
evalml/pipelines/pipeline_base.py	100.00% <0.00%> (+6.32%)	⬆️
.../automl_tests/test_automl_search_classification.py	100.00% <0.00%> (+10.44%)	⬆️
... and 16 more

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[freddyaboulton]

@angela97lin Looks good to me! Left some minor comments. Nothing blocking.

[bchen1116]

LGTM!