Update Overview section

[robertswh]

This review is to check the content of a couple of new articles and some changes in a third article. There is no code to run so you can review on GitHub. If you want to clone the repo and build the book you can follow the instructions in the README, however installing the requirements can take a while so I suggest reviewing the markdown files on GitHub.

Checklist:

• [x] Getting Started with Spark - read through and look for any obvious issues, make suggestions for improvement
• [x] When to Use Spark - read through and look for any obvious issues, make suggestions for improvement
• [x] Guidance on Spark Sessions - check the changes to this make sense, make suggestions for improvement

The idea behind the changes for the third article is to make it more general and not as specific to DAP.

[PeterJDerrick89]

@robertswh I have reviewed the changes, they look good to me. It wouldn't be appropriate for me to merge this in. The convention I tend to follow is, after a peer review has been done by someone else and they are happy with the changes made, the person who made the changes or the tech lead merges the changes into the main branch.