Docs - Getting started: refactor 'Customizing Bootstrap'

[StevenBlack]

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.)

[cvrebert]

"include" would make more sense than "invoke", IMO. CSS isn't a programming language per se.

[mdo]

This and L53 shouldn't include a .

[StevenBlack]

Done!

[cvrebert]

Need to remove the dot here too.

[StevenBlack]

Done (sorry I missed that. Found another actually.)

[Merg1255]

Providing a screenshot of the final product would be nice.

[StevenBlack]

Here you go, well, 90% of it. My capture tool doesn't scroll from an arbitrary point.

[woshicixide]

good

[Merg1255]

It looks nice.

[StevenBlack]

• Ping *

I'd like to further develop this section, building upon its latest state, in a new branch.

In other words, basic cleanup (this PR) first, then its extension with more advanced workflows, which are surely a separate discussion from this first-pass cleanup.