DOC: Add RangeIndex to the documentation and fix warnings

[datapythonista]

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.

[jorisvandenbossche]

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

Yep, indeed. Your PR looks fine for that.
Only not sure about the removal of the attributes from the RangeIndex docstring. To get those index specific attributes in the API docs, we need to list them manually (when you use the "class_without_autosummary" template)

[jorisvandenbossche]

This is fixed in the meantime.