DOC: Reorganize Getting Started documentation pages
- [x] closes #32308 

Issue Related:
- `getting_started/index.html` removes `10 minutes to pandas`, `Essential basic functionality` and `Intro to data structures` from the toctree and moves them as links in `user_guide/index.html`
- `getting_started/tutorials.html` now only references community tutorials
- the pandas cheatsheet is now referenced at the bottom `Tutorial` section of `getting_started/index.html`

Misc:
- Lowercases `Pandas` to `pandas`
- Some rephrasing

My sphinx-foo is a bit poor.

I see I'm getting these doc build warnings

```
2020-03-02T07:41:43.8295810Z /home/runner/work/pandas/pandas/doc/source/getting_started/10min.rst: WARNING: document isn't included in any toctree
2020-03-02T07:41:43.8296699Z /home/runner/work/pandas/pandas/doc/source/getting_started/basics.rst: WARNING: document isn't included in any toctree
2020-03-02T07:41:43.8297521Z /home/runner/work/pandas/pandas/doc/source/getting_started/dsintro.rst: WARNING: document isn't included in any toctree
```

And we have a note in the docs

```
.. If you update this toctree, also update the manual toctree in the
   main index.rst.template
```

But still unsure what I need to change in `index.rst.template` cc @jorisvandenbossche  @mroeschke for the warnings, I think you need to add a toctree to `tutorials.rst` with those (10min, etc.) documents included. Otherwise they won't be included in the output. Or alternatively put `:orphan:` on the top of the rst page that is not included in any toctree but we still want to include in the output, which will silence the warnings as well.

However, I think we want to include it in a toctree for better visibility? Yeah, I think including it is better. I think putting it in an `:orphan:` page also kinda breaks "next" and "previous" links. I am not sure about moving 10min to the user guide. In any case, based on its name I would say it belongs in the getting started section.

cc @datapythonista are you OK with moving dsintro.rst and basics.rst back to the user guide? (since you put them in the getting started section when the split was done) Planning on merging tomorrow if there are no further comments.