DOC: Update of the 'getting started' pages in the sphinx section of the documentation
- [ ] closes #26831
 
This PR provides an update of the getting started pages of the Pandas documentation, following from the discussion in #26831 and the [proposal](https://docs.google.com/document/d/1Rc_eql5KLrdf0c582KyWfs2ADVNxbJy4jfosnqdrVak/edit#heading=h.mqz2f6gbl3sd). The update went together with the creation of the new theme and tries to integrate as much as possible with the new layout (using bootstrap elements).  This PR focuses on the getting started section and adds new sections to the documentation:

 - An update of the 'getting started' intro page of the sphinx documentation to provide new users some more guidance on the 'first steps' with Pandas: Directions for installation, a short intro on main Pandas features each linking to a dedicated introduction tutorial (see next point), specific info targeted to people with another background and reference to more tutorials.
- A set of tutorials which introduce some key features of the Pandas package. Each tutorial is setup as a series of 'tasks/questions' so users get a first idea of the type of problems they can solve with Pandas for that topic. All tutorials provide a set of [returning elements](https://stijnvanhoey.github.io/pandas-getting-started-tutorials/tutorial-elements/). Both the intros as well as the tutorials itself try to guide people as much as possible to related sections of the user guide. 
- A set of schemas to illustrate certain concepts, with according to the new theme. These schemas can be used in other locations in the user guide as well.

The aim is to move the [dsintro](https://pandas.io/docs/getting_started/dsintro.html) and the [basics](https://pandas.io/docs/getting_started/basics.html) to the general user guide, as both are too extensive for a getting started section. The 10 intro tutorials try to provide an alternative that is more fit to people starting with Pandas. 

There is still some work to do (typo's in the tutorials, additional schemas...), but input is certainly welcome and appreciated, I'll continue improving it. See https://stijnvanhoey.github.io/example-pandas-docs/getting-started/getting_started/index.html for a live preview.

Note: (1) a separate PR is prepared for the general sphinx intro page with @jorisvandenbossche, see #31148  (2) some of the layout issues are more general and related to the [new sphinx theme](https://github.com/pandas-dev/pydata-bootstrap-sphinx-theme) and will be tackled there (e.g. spacing around titles and subtitles)

Some pictures:
- The intro to the 10 tutorials:
![image](https://user-images.githubusercontent.com/754862/72739475-fbe4b800-3ba3-11ea-9256-1617ed726b44.png)

- the coming from section :
![image](https://user-images.githubusercontent.com/754862/72739547-1c147700-3ba4-11ea-9cda-e518af641df7.png)

- example of 'remember' section in each tutorial:
![image](https://user-images.githubusercontent.com/754862/72739624-4403da80-3ba4-11ea-81b8-0df26fc129f7.png)


 