The Documentation Tools palette provides a set of tools for working with Mathematica documentation. There is also a set of functions, but typically these are called either by the Workbench or by the palette.
To open the palette you should open a notebook from a paclet project and then look in the Palettes menu of the Mathematica notebook front end. When it opens it has a number of tabs, one of which is shown below.
To get more information you should see the DocumentationTools section of the documentation.
When you open the Documentation Tools palette, you may see a dialog asking you to confirm loading the palette, as shown below.
You should click "Enable Dynamic" in order to use the palette. If you want to have the palette always trusted by front end, which will suppress this dialog, there is a tool to do so in the paclet preferences.