Site Instructions are notes for the people who maintain this site — how to add a page, which template to pick, where to put images, and similar day-to-day guidance. They are not a replacement for a package’s public documentation page. Package authors can still ship instructions with a package so installing the package brings the notes along.
With a site open, use the collapsible Site Instructions tab on the right. If a topic is already selected, that topic opens by default. Otherwise you see Manage Topics.
You can also search the public help site from the same dialog when you need Marketpath documentation rather than this site’s notes.