rename "Open Neuroimaging Working Group" to "Open WIN Steering Group"

[cassgvp]

As the individual tools teams grow, we need a way to distinguish their activity from the group of WIN PIs who have been involved in the project from the outset.

In line with the alterations which will be made on the main WIN website, we will be make two changes to the naming conventions we use in referring to individual teams on these pages:

1. The "Open Neuroimaging Working Group" will become the "Open WIN Steering Group"
2. The "Tools Teams" will become the "Tools Working Groups"

All references to these groups will need to be updated in this repo.

Figures which are named with "[...]-onwg-[...]" will also need to be re-drawn with correct text. Images are created and can be exported from the file https://github.com/cassgvp/WIN-Open-Neuroimaging-Community/blob/master/docs/img/figures.key

Images to be redrawn:

• https://github.com/cassgvp/WIN-Open-Neuroimaging-Community/blob/master/docs/img/img-onwg-h250.png
• https://github.com/cassgvp/WIN-Open-Neuroimaging-Community/blob/master/docs/img/img-onwg-narrow.png
• https://github.com/cassgvp/WIN-Open-Neuroimaging-Community/blob/master/docs/img/img-onwg.png