DOC: Final reorganization of documentation pages
- [X] xref #24499

You can see the changes here: https://datapythonista.github.io/pandas-doc-preview/

Pages moved:
- comparison_with_whatever -> getting_started/comparison/comparison_with_whatever
- ecosystem -> getting_started/ecosystem
- cookbook -> user_guide/cookbook
- rpy2 has been removed

Changes to the documentation home page:
- Moved the long description to the Package overview page, and left a small paragraph, and a link to it
- Made the toctree show just 2 levels, so it's useful and it can be seen what it's there

May be useful to see how the master docs look like, to compare: https://pandas-docs.github.io/pandas-docs-travis/

I will follow up with:
- A change of the style (the one we saw in the Austin sprint)
- An integration with the pandas website (pydata.pandas.org). I'll create a issue for this before, as there is some discussion needed.

@TomAugspurger @jorisvandenbossche @jreback 


