Add details on how to write fog Mocks
Be consistent about capitalising Mock

Though fog is definitely with a lower case f, so this looks odd, oh well.
Clarify with structure

This commit adds a bit more structure. It also makes a couple of corrections to tone (e.g. 'we've' instead of 'I've' given this is now on a public website, and it adds a link to the PR I made to give an example.
Merge pull request #195 from gds-operations/fog-mocks

[#72073602] Add details on how to write fog Mocks