Add vignette with top-level walkthrough on what functionality to add to a DfE dashboard

[rmbielby]

Just thinking analysts would benefit from a step-by-step walkthrough of what they are expected to add to their dashboard using dfeshiny. It would reference other vignettes where needed, rather than duplicating anything (e.g. the cookies vignette).

[cjrace]

Agree - think this will be really helpful