Add runbooks as category to release

This introduces runbooks as a new category for release in 2019, with https://gitlab.com/gitlab-org/gitlab-ce/issues/56091 as the first MVC to move forward with this feature. There are a few reasons why I think this is its own category, rather than a bullet point under release orchestration:

• Runbooks can also solve other operational needs; release orchestration problems can be solved here, but so can other interesting things
• Introduces new internal customer (production team)
• Also competes in new spaces against different kinds of products

I've placed this under the 'supporting functions' subgroup of Release, which I think this makes sense there as a standalone product category and also balances things more equally.

Review App Links

• https://jlenny-runbookscategory.about-src.gitlab.com/handbook/product/categories/#release
• https://jlenny-runbookscategory.about-src.gitlab.com/direction/release/#categories

To-DO

If this is approved, I will need to:

• create category vision epic for runbooks
• create a few more follow up items for improvements to include in category vision
• update existing issues as appropriate