Move Monitor docs to separate top-level menu

The Monitor docs have been separated out across multiple places, and not organized in a way that worked well for the Monitor team. This MR takes the existing documentation for Monitor and moves it into a new top-level menu item, "Operations."

Many items are missing, because they are all jammed into the giant (7000-word) Prometheus page. As pages are split out of the Prometheus page, slimming it down, they'll get added to the Operations menu.

Related to gitlab#227441 (closed) which outlines the full scope of the work.