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