Rename directory 'doc' to 'docs'

Since docs seems to be the standard directory name for documentation, we should use that instead of doc.

See:

• https://help.github.com/en/github/building-a-strong-community/creating-a-default-community-health-file#about-default-community-health-files
• https://docs.gitlab.com/ee/user/project/repository/web_editor.html#create-a-directory
• https://help.github.com/en/github/building-a-strong-community/setting-guidelines-for-repository-contributors