DOC: Add RangeIndex to the documentation and fix warnings
xref #24173

Looks like we don't have `RangeIndex` in the documentation, and this is causing warnings:
```
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.DataFrame.sparse'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.T'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.array'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.base'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.data'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.dtype'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.dtype_str'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.flags'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.hasnans'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.inferred_type'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.is_all_dates'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.is_monotonic'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.is_unique'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.itemsize'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.nbytes'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.ndim'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.shape'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.size'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.start'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.step'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.stop'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.strides'
pandas\doc\source\reference\index.rst:: WARNING: toctree contains reference to nonexisting document 'reference/api/pandas.RangeIndex.values'
```

In https://github.com/pandas-dev/pandas/pull/26786/files I'm adding the class, but still will require the methods.

From similar classes I see that we just add to the autosummary the methods specific to the index classes, and not the ones inherited from the parent `Index`. I guess we want to do the same here, but we need to make sure there are no warnings related to `RangeIndex` in the docs build.

We also want to make sure the docstring of `RangeIndex` is correct, and has the appropriate attributes and methods.