Consider deprecating or confirming when to use "additional page metadata"

Following on a discussion from !34952 (merged):

• @axil started a discussion: (+1 comment)

  We used to use article_type for tutorials, we might need to deprecate it, since we also have type now.

Let's consider the whole list under https://docs.gitlab.com/ee/development/documentation/#additional-page-metadata.

The following metadata parameters are available. Those that should be removed should be selected from the list:

• author
• author_gitlab
• date
• article_type
• level
• last_updated
• reading_time