Refactor handbook.js to generate-navigation-helpers.js

Why is this change being made?

To help improve clarity around "handbook" specific JavaScript use and more general use. In this case, most "handbook" templates (as defined here) want the following:

• table of contents generation
• header linking + associated scroll behavior

Other templates, namely security.haml want this behavior too (and potentially others). By refactoring the file name, function names, and leveraging the promise API to enforce intentional sequence of function execution we add clarity of expectation in addition to reuse opportunity.

• I initially considered breaking this script in to smaller files, but came to learn we don't leverage a bundler and this would result in more requests so I opted to just refactor for now.*

Author Checklist

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