Modify `Zero` discussion

[chiphogg]

• Add explicit downsides to function approach section.
• Zero has only one downside, which manifests in different ways.
• Delete CTAD example, because who would write quantity q2 = ZERO;?
• Delete for-loop example; we've already discussed it's not important.
• Clarify MSL example by showing what would not work.
• Move refactoring example higher as it's more important.
• De-emphasize multiplicative example, as users aren't motivated to write that anyway.
• Fix numbering.
• Tweak closing statement.

[mpusz]

OK, let's merge it as it is. MAybe I will apply some small fixes by myself if I have time before the deadline.

[mpusz]

Thanks Chip! Have a great Sunday with your family! It was a huge effort for all of us. But we made it!