FAQ for Hamilton

**Is your feature request related to a problem? Please describe.**
We could probably make finding answers to common questions a lot easier. Each should have a 2-sentence TL;DR with pointers to more documentation:

1. Caching/workflow restart
2. How to do filters/joins with series
3. How to handle spark dataframes
4. Approaches to scaling up
5. Ways to configure your pipeline

**Describe the solution you'd like**
An FAQ page in the docs/maybe a .md page in the repo (they can be the same).

**Describe alternatives you've considered**
I think this is worthwhile to help onboard/appeal to new users.