arrange sections of cookbook and add some introductory content

[jacobvjk]

relates to #104

• cookbook.Rmd gains a section "Overview" that covers:
  • What is the PACTA for Supervisors analysis?
  • Who is this tool built for?
  • What can the results of the PACTA for Supervisors analysis be used for?
  • What are the main steps of the analysis?
• this section is meant to give a high level overview of what the reader can expect from the tool in terms of analysis and from the cookbook in terms of documentation
• Moves other sections around to fit the structure of the cookbook outlined in the related GH issue

NOTE

• this PR is a draft version of the "Overview" section. The final version may look different and will consider substantial reviews.
• To be clear, this PR only addresses the first checkbox of the linked GH issue

[jacobvjk]

@jdhoffa @cjyetman I am sure the overview section can be improved, but this is a first draft that I think covers many of the points at a high level that we are aiming to achieve with the cookbook. Unless you see any major issues with the content, I would suggest to prepare the draft first and improve each of the sections with inputs from our non-tech colleagues later