locations of docs wrong when built using staticpkg #313

Describe the bug
static page has wrong links to some static docs like

1. Pair device help page goes to localhost/device/usb when it should be localhost/docs/device/usb
2. Go to locally hosted editor > help > about > open in new window
   

the page links to http://localhost/docs/docs/about.html

To Reproduce
Steps to reproduce the behavior:

1. ollow all the steps in Local Dev Server in pxt-maker github page.
2. use command pxt staticpkg to generate static page
3. create a new project select any board with usb host supported like feather M0 express.
4. click pair device icon and then click on help
5. See error

Desktop (please complete the following information):

• OS: ubuntu 18.04 LTS
• Browser chrome
• Version 81.0.4044.122 (Official Build) (64-bit)