[MNG-1893] Update handbook to reflect new "core committer review" Jira status

https://tasks.opencraft.com/browse/MNG-1893

I have now added a Core Committer Review status column. It will show up in on boards in the same column as “Internal Review”, since it’s a type of internal review. Along with the new "Core Committer" assignee field, this allows core committers to easily set up Jira filters/searches that show their upcoming core committer reviews, as well as PRs that are currently waiting for their review.