Add basic example for collapsible components

[ianw]

Currently the "collapse" documentation jumps straight into the
accordion example and then mentions in passing the slightly simpler
example of using a single component to expand/collapse a region after
that.

This leaves some room for misunderstanding; for example I spent a
while not realising that I'd set data-target in my links, which
makes the page jump around when clicked as the default action isn't
prevented (see [1]).

This proposes a simple example first up, using a link and a button to
expand a div. It also explicitly mentions use of href and
data-target. The more involved accordion example then follows
unmodified.

[1] #2001

[mdo]

Something simpler would be awesome, and something even simpler than a list group would be even better. A basic button toggle to show another element would be enough I think.

[ianw]

@mdo I think the button example is good, but the other point of this proposed change was to also highlight the difference between usage of href and data-toggle. I think that's worth calling out and I'm not the first person to have made that mistake

[mdo]

@ianw Oh word, misread that part. Perhaps you could open a PR to extend the example I added to include two separate buttons?