Docs - Getting started: refactor 'Customizing Bootstrap'

There's still much to add to the "Customizing bootstrap" section.  This is just a starter: normalizing what's already there for readability, keeping non-native speakers in-mind.  In some places I added minor clarifications.

Notable changes here: 

* Shrank and normalized section whitespace breaks.  It's three linefeeds everywhere now.  Not sure if there's a standard for this, but this looks fine to my eye.

* Most changes make shorter, less wordy sentences.  There are exceptions where I felt clarity trumped brevity.

Looking forward, we should distinguish between 'Customizing Bootstrap'" (modify and extend as in this section), and "Customize and download" (theming and cherry-picking components and features with the wizard.)