Update docs

@cmason (please review)

- Update sidebar layout 
- Update application services page
- Update subscriptions page with separate gists
- Add separate pages for push and vault w/ associated gists
- Add Notify Me (push) sample
- Fix some typos and broken links
- Group all content into 4 buckets (dev portal, docs, samples, and demos)
- Change URL design to make more sense when we have an Android SDK