Support use of Docsy as an NPM package

[chalin]

This is a tracking issue for the support of using Docsy as an NPM package. This is already possible

npm install https://github.com/google/docsy

but it isn't practical until #948 is implemented too. Also, it is not yet an officially supported ways of using Docsy -- but I'll bring it up for discussion in 22Q2.

(For more detail concerning the installation options of NPM packages from GitHub, see Install Packages From GitHub.)

• Code changes - Support project-level NPM modules for BS and FA #1117
• Update the docs - Document use of Docsy as an NPM package #1119
• Update the CHANGELOG - CHANGELOG: mention new docsy-as-NPM-package option #1125