DOC: Reorganization of the documentation
The documentation currently has almost all the pages in the same level. Making it difficult to navigate.

Having few top-level sections that group the pages, and some hierarchy would make things simpler to find. Here there is a prototype of how having few top-level sections could look like: https://pandas-dev.github.io/pandas-sphinx-theme/pr-datapythonista_base/

Those are the pages we currently have:
```
10min
advanced
api
basics
categorical
comparison_with_r
comparison_with_sas
comparison_with_sql
comparison_with_stata
computation
contributing_docstring
contributing
cookbook
developer
dsintro
ecosystem
enhancingperf
extending
gotchas
groupby
indexing
index
install
internals
io
merging
missing_data
options
overview
releases
reshaping
r_interface
sparse
style
text
timedeltas
timeseries
tutorials
visualization
whatsnew/
```

An initial proposal of organization is:
```
index

install

getting_started (probably making this a 3 or 4 part tutorial is a good idea)
    overview
    10min
    basics
    dsintro
    tutorials (I'd remove this page)
    comparisons
        comparison_with_r
            r_interface (I'd remove this page)
        comparison_with_sas
        comparison_with_sql
        comparison_with_stata

user_guide
    io
    indexing
    advanced
    merging
    reshaping
    text
    missing_data
    categorical
    visualization
    computation
    groupby
    timeseries
    timedeltas
    style
    options
    enhancingperf
    sparse
    gotchas
    cookbook (not sure if user_guide is the right place, but I don't see a better one)
    ecosystem (not sure if user_guide is the right place, but I don't see a better one)

api
    splitted in #24462, names of secions and possible changes to structure to be discussed 

development
    contributing (I'd split this one)
        contributing_docstring
    extending
    developer (this could probably be merged with extending)
    internals

whatsnew/
    releases -> index
```

@pandas-dev/pandas-core thoughts?