docs: improve js files info

Created by: ducin

There are some people that fall in trouble with including both bootstrap.js and bootstrap-*plugin*.js files at the same time. It's missing in the docs, that the main .js file includes all - or at least it's not clear enough. For example, look at this stackoverflow issue.

I suggest to emphasise in the getting started page, that if the developer includes the main .js file, it's enough and no other plugin files shall be included separately.