cherrypick from mnc-io-docs docs: Updated Billing API documentation

(for Project Nickel).

This CL is a continuation of CL 1026559.

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: Id194c17b5dd23fc89b9b69cf90e601993a66d014
Original-Change-Id: I917d06876d1525f24cb26a55a39cd64ae479b8f8