merged Custom Elements 101 with platform/custom elements and updated platform docs for new nav
incorporated Arthur's feedback
Merge pull request #757 from Polymer/platform_docs

merged Custom Elements 101 with platform/custom elements and updated pla...
Merge branch 'master' of https://github.com/Polymer/docs into wc-reorg

Conflicts:
	elements/docs-menu.html
Add redirect for old custom elements 101 doc.
More cleanup.