Docs - Getting started: refactor 'Customizing Bootstrap'
Better wording, avoiding 'invoke', and misc edits
Standardizing on LESS (uppercase) and avoiding the word 'less' where that's easy and sensible.
Removed the '.' from '.LESS' in two instances.
Fixed two more instances of [.]LESS in the text
fixes #10491: new docs background base64 image
Merge branch 'GSEdits7' of https://github.com/StevenBlack/bootstrap into StevenBlack-GSEdits7

Conflicts:
	getting-started.html
fix broke width of containers in docs given change to use width over max-width
Merge branch 'master' of github.com:twbs/bootstrap