Add a example to illustrate the relationship between award values, contract values and amendments

[duncandewhurst]

From @yolile in #1534 (comment):

Sounds good. For contract values, a real example was discussed in #575 (comment), and we had already agreed that the award vs. contract value should work for that.

But I'm wondering if we should add a worked example or similar to the documentation to make this clearer (and maybe check our data use guidance around contracts overrun/budget indicators to ensure we are also suggesting to compare awards vs. contracts for calculating them)