Add a new section explaing how to customize figures

[nzw0301]

Motivation

Resolve #5178

Description of the changes

Add a new section to clarify the methods that return editable objects and a simple example to replace titles of plot_intermediate_values.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (308a645) 89.40% compared to head (da447bd) 89.36%.
Report is 29 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5194      +/-   ##
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- Coverage   89.40%   89.36%   -0.05%     
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  Files         206      206              
  Lines       15126    15097      -29     
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- Hits        13524    13492      -32     
- Misses       1602     1605       +3     

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[nzw0301]

The link to the corresponding page section is
https://optuna--5194.org.readthedocs.build/en/5194/tutorial/10_key_features/005_visualization.html#customize-generated-figures.

[HideakiImamura]

How about adding the link to the document of Plotly?

https://plotly.com/python-api-reference/generated/plotly.graph_objects.Figure.html

In addition, I think it would be great to add an example for Matplotlib. Could you add that?

[nzw0301]

Thank you for your feedback. Sure I'll update this PR in a few days.

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[HideakiImamura]

Hi @nzw0301. Is this PR ready for review?

[nzw0301]

@HideakiImamura Yes! 19121aa includes your suggestion.

[nzw0301]

Sorry, I realised that plotly's link is broken, concretely, the generated link is https://plotly.com/python-api-reference/generated/plotly.graph_objs.Figure.html#id0 which is not found. Note that this broken link problem also happens in pages of optuna.visualization. I'll fix the link for this tutorial.

[HideakiImamura]

LGTM.