Add examples of newly added feature to the documents

[Alnusjaponica]

What is an issue?

Motivation

Follow up #4986 and #4871

Details

Use newly added feature in the examples written in docstrings. To be specific,

optuna/optuna/visualization/_param_importances.py

Lines 123 to 144 in 2f279f3

	Example:
	The following code snippet shows how to plot hyperparameter importances.
	.. plotly::
	import optuna
	def objective(trial):
	x = trial.suggest_int("x", 0, 2)
	y = trial.suggest_float("y", -1.0, 1.0)
	z = trial.suggest_float("z", 0.0, 1.5)
	return x ** 2 + y ** 3 - z ** 4
	sampler = optuna.samplers.RandomSampler(seed=10)
	study = optuna.create_study(sampler=sampler)
	study.optimize(objective, n_trials=100)
	fig = optuna.visualization.plot_param_importances(study)
	fig.show()

sould be rewritten or have new examples.

Also, one of the tasks described in #4871 (comment) seems not resolved.