merged Custom Elements 101 with platform/custom elements and updated pla...

...tform docs for new nav

Merged  platform>customelements with custom elements 101. Putting the new doc under "About Custom Elements" in the new side nav world. Also removed polyfill details from the platform docs and pulled them out of the submodules (removed include_external).

Because I merged two docs, I needed to remove custom elements 101 from side nav. Need a redirect from /docs/start/customelements.html > /platform/custom-elements.html

Thanks.
mem
This one is the odd one out, since it's not contained in the web components polyfills and is only included with one (or some of) the elements.

So we might want to leave on the spec header for now until we figure out what to do with this one.