CLN:replace single with double backticks

[satrio-hw]

#36617

doc/source/development/contributing.rst
doc/source/development/contributing_docstring.rst
doc/source/development/extending.rst
doc/source/ecosystem.rst
doc/source/getting_started/install.rst
doc/source/getting_started/comparison/comparison_with_sql.rst
doc/source/getting_started/intro_tutorials/06_calculate_statistics.rst
doc/source/getting_started/intro_tutorials/08_combine_dataframes.rst
doc/source/user_guide/categorical.rst

[dsaxton]

@satrio-hw The doc build for this seems to be failing, note that cross-referencing syntax like :class:`Series` does use single backticks: https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#xref-syntax

[dsaxton]

Thanks @satrio-hw for the PR but the issue is now fixed