Created and added content to Getting Started page

- How-to guide and gallery links have been disabled pending implementation. (have updated related jira issues)
- Created a ```disable-link``` CSS rule. 
- Introduced "API" and "How To Guide" badges (we may want to roll these out over the site)
- Updated "Getting Started" link on site, no longer points to GitHub
We should add target="_blank" to the d3 and crossfilter links.