docs(home): move commit link above the fold; add link to github issues search

Move the paragraph with the commit hash above the video
 \- this was below the fold on a 768px screen, making it easy to miss
Reword Github information
Add a link directing users with problems to the github issues search
 \- this shows users where to look for solutions before creating a new issue

Resolves https://github.com/angular/material/issues/566

![material-docs-fix-566](https://cloud.githubusercontent.com/assets/7917920/4962672/b62eedb4-66e5-11e4-85bf-d01bafd646b5.gif)