Add getting started notebook

[cthoyt]

As a follow-up to #140, this adds/updates some additional tutorial notebooks

• Make Counterfactual Graph
• Getting Started

[codecov]

Codecov Report

Merging #148 (dc40d48) into main (22cdac4) will increase coverage by 2.46%.
The diff coverage is n/a.

❗ Current head dc40d48 differs from pull request most recent head 9f2665f. Consider uploading reports for the commit 9f2665f to get more accurate results

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   78.07%   80.54%   +2.46%     
==========================================
  Files          36       39       +3     
  Lines        2176     2416     +240     
  Branches      481      550      +69     
==========================================
+ Hits         1699     1946     +247     
- Misses        381      387       +6     
+ Partials       96       83      -13     

see 10 files with indirect coverage changes