docs: Updated Billing API documentation (for Project Nickel).

Added references to Project Nickel (rounded and "relevant" pricing
for each country where an Android app is published) within overview
and reference pages. Also added discussion of pricing templates and
the associated operations in the "administration" page.

Finally, completed minor touch-ups to billing administration section
(removed unhelpful links, updated screen captures).

Bug: 28204935
Change-Id: I917d06876d1525f24cb26a55a39cd64ae479b8f8