Improve the Your First Contribution section of contributing.md

[aburdenthehand]

As pointed out in #968, the Your first contribution section in the Contributing guide is not particularly clear.
It would benefit from a rewrite and to actively link to the easier or most common repos (user-guide, kubevirt.github.io, community, kubevirt, CDI etc). We should also link to the New Contributor session video we recorded recently.