Add a maintainer step to hand-test packages

See the general Documentation guidelines https://docs.gitlab.com/ee/development/doc_styleguide.html

What does this MR do?

Add a maintainer step to CONTRIBUTING talking about the value of hand-testing packages

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page
• Change the header tag to h5, #####

Moving docs to a new location?

See the guidelines: https://docs.gitlab.com/ee/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.