docs(dropdown): align the responsive alignment examples to the toggle

[xidedix]

Description

With responsive alignment Popper is off and the menu is placed by CSS, relative to the dropdown itself. The default variant renders that as a full-width block, so both examples dropped the menu against the edge of the container instead of the button. They now set variant="btn-group", which wraps just the toggle, and the page says why it is needed.

Two more fixes on the same section:

• The first example used alignment="end" and the label of the menu alignment example, so it never demonstrated responsive alignment at all. It now uses alignment={{ lg: 'end' }} with the matching label.
• The prose named an aligment prop that does not exist, in a syntax the component never accepted.