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
Since this isn't a polyfill doc anymore, replace the subtitles with a one-line description. You could just use an abbreviated version of the "summary" below -- "Define and use new types of DOM elements in a document."