Improve `index.html` content

[julien-deramond]

This PR is heavily draft and is not ready for a full review yet. It is meant to be a starting point for discussion (/cc @mdo).

I find the current index.html content confusing as we explain that the user has succeeded to load the example with the default button rendering, but we also talk about a customized purple link (which on top of that is the default color of visited links). It's not that obvious IMO that we also customized some things in this example.

The idea here is to have two separate sections:

• the first one shows the default rendering of the button and the link
• the second one shows that the danger color can be customized and that the link and button can be customized as well. Chose deep pink to show that obviously, something changed, and that is not the default color, nor comes from Bootstrap.

If this idea is good and accepted, all the other examples should be updated to follow this pattern.