Re-order documentation for astropy.units - #1591

@astrofrog - we're of t̶w̶o̶ one mind (see #1577 (comment))! Though my initial solution would have been to move the quantity part to its own subsection in the master index. But I like your suggestion better: start straight with 15 * u.m / u.s; in that case, just rename the section "Quantities and Units"?

@mhvk - your last suggestion sounds good - at the end of the day I agree the Quantity and Unit stuff should live together and I like the title you suggest.

See the overlap with #1595 - if we do that, this is not necessary because it would have it's own doc section.

I do not think we should give it it's own section in the doc (as @mhvk originally suggested) unless it has its own subpackage. But I'm also fine with @mhvk's suggestion here to actually put the quantity stuff before the units stuff in the docs (and more prominent in the examples). So now that I see this, I'm actually slightly more in favor of this over #1595.

I'm also not too keen on actually separating units and quantity, so I also prefer the solution I suggested here.

Self assigning. I'll do this work early next week.

Reorganization of units document to emphasize Quantity is attached.

It primarily:

1. Presents Quantities first
2. Updates many of the examples demonstrating unit concepts using quantities
3. Removes some now incorrect documentation about the Quantity conversion convenience members (i.e. q.parsec).

This requires #1610 to be merged as well, as it documents a feature there.

I went through all the files and it looks good.

Apart from the simple comment above, maybe for the cds format mention that it underlies every table that one can get from Vizier [1] -- maybe more relevant than that it was used in early votable definitions!

[1] http://vizier.u-strasbg.fr/

I have addressed @mhvk's comments and (hopefully) fixed the Travis errors.

👍