Flyout: investigate position=inline

[humitos]

It would be good to be able to explicitly define <readthedocs-flyout position="inline"></readthedocs-flyout> in the HTML and make it behave "inline". That means that's not going to be floating, but adapting itself to the container it's inside, respecting its width.

I don't know exactly how to write the CSS for this, but this issue about exploring this idea and collect any knowledge that may be needed to achieve that goal.

References:

• https://sphinx-rtd-theme.readthedocs.io/en/stable/configuring.html#confval-flyout_display
• Please provide a way for Sphinx themes to "take over" the flyout menu readthedocs.org#11688