Flushed out structure for the plugin section of the guide

The name of the branch is misleading; I started to write the type def section and realized I wanted to see the higher context of where it'd fit in. In doing so, I stepped back across all three plugin types and established a common pattern based on how I was doing the server docs.

The inclusion of an installation page in each of the three components might change. I'll take a look at restructuring the entry points page (which currently lives under extensions) into something that all three of them can refer to to prevent each from rewriting it. It may be that the installation page of each section says the specifics for its section (i.e. the entry point namespace) and says "Look here for more info on entry points".