Add a vignette for creating a new DfE R project

[cjrace]

Is your feature request related to a problem? Please describe.

We should add a walkthrough guide to using the create_project() function.

Describe the solution you'd like

Something aimed at someone with very little knowledge of R, to walk through the ways to use it and any decisions they need to make. Will also be a nice chance to introduce users to R projects, R studio and renv.

Describe alternatives you've considered

Not put much thought to other alternatives, I guess we could do an analyst guide addition, but it will probably be nicer as a self-contained page on the package documentation (and encourages others to look at the package documentation).

Additional context

Lots of notes about the function and it's usage in #71 that can be recycled.