Updates to existing Governance Guidance

One image removed (no longer relevant). Adjustments to comments based
on feedback from content initially published. Content written by Governance team and content designed by Beck Thompson
[Contrary to style guide](https://www.gov.uk/design-principles/style-guide/style-points#style-dates-and-times)

[New version concurs](https://www.gov.uk/guidance/content-design/writing-for-gov-uk#date-ranges)