Add keyword explain to private vizro-ai method

[maxschulz-COL]

Description

Currently the docs example:

import vizro.plotly.express as px
from vizro_ai import VizroAI

vizro_ai = VizroAI()

df = px.data.gapminder()
code_string = vizro_ai._get_chart_code(df, "describe life expectancy per continent over time", explain=True)

fails due to the keyword explain not being present.

Given that users might want to extract the code, explanation and insights separately, it makes sense to allow this keyword AND to also allow the return of the entire dictionary containing the code string, explanation and insights (see updated docs example).

Screenshot

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[Anna-Xiong]

approved already but we need to change it to True to get explanation otherwise it is none for explanation.