provide more docs for `dash_duo`

[janosh]

dash_duo seems to have a lot of methods like select_dcc_dropdown that I couldn't find any docs for?

dash/dash/testing/browser.py

Line 409 in 04217e8

def select_dcc_dropdown(self, elem_or_selector, value=None, index=None):

The best resource I found is https://dash.plotly.com/testing#browser-apis. Are more comprehensive dash_duo docs on the roadmap or already in existence somewhere?

[alexcjohnson]

Oh interesting - looks like we documented many of these but the docstrings are copied in, not synced with the current version of Dash. @LiamConnors maybe worth a new markdown directive to create these docs pages from dir(object), like we currently do in code for https://dash.plotly.com/reference?