Improve the documentation page

[EricSmekens]

I've recently updated the documentation page for version 1.4.0:
https://ericsmekens.github.io/jsep/

The sandbox and links are properly updated, but the documentation itself is out of date.
(There is no guide on how to use plugins for example).

The documentation is currently edited and published from the gh-pages branch. My feeling is that this can be improved, and we might want to re-use documentation that is used in the README.md.

This is just a placeholder so I (or anyone else) can improve this situation.
Suggestion on how to tackle this and maybe to modernize this as well, is welcome.

Related to #134