DOC: Consolidate pages under /docs/getting_started
Another piece of feedback from the CZI EOSS kickoff meeting that @WillAyd and I attended:

In https://pandas.io/docs/getting_started/index.html, we link to the following pages in the left side:

<img width="1429" alt="Screen Shot 2020-02-27 at 11 09 33 AM" src="https://user-images.githubusercontent.com/10647082/75478083-2e779280-5952-11ea-9cf9-d49c308a69f1.png">

We got feedback to:

1. Have a dedicated link to the pandas cheat sheet: https://github.com/pandas-dev/pandas/blob/master/doc/cheatsheet/Pandas_Cheat_Sheet.pdf
2. Consolidate `Getting started tutorials`, `Tutorials`, and `10 minutes to pandas` (which is linked already in `Tutorials`) into a singular `Tutorials` page
