CI: Add rst backtick checker

[dsaxton]

Adding a pre-commit hook for detecting single backticks around code in RST files. Running for instance on the v1.2.0 whatsnew shows errors:

doc/source/whatsnew/v1.2.0.rst:120:- `Styler` now allows direct CSS class name addition to individual data cells (:issue:`36159`)
doc/source/whatsnew/v1.2.0.rst:225:- Performance improvements when creating DataFrame or Series with dtype `str` or :class:`StringDtype` from array with many string elements (:issue:`36304`, :issue:`36317`, :issue:`36325`, :issue:`36432`)
doc/source/whatsnew/v1.2.0.rst:230:- Performance improvement in :meth:`pd.to_datetime` with non-`ns` time unit for `float` `dtype` columns (:issue:`20445`)
doc/source/whatsnew/v1.2.0.rst:265:- Bug in :func:`date_range` was raising AmbiguousTimeError for valid input with `ambiguous=False` (:issue:`35297`)
doc/source/whatsnew/v1.2.0.rst:306:- Bug in :meth:`SeriesGroupBy.transform` now correctly handles missing values for `dropna=False` (:issue:`35014`)
doc/source/whatsnew/v1.2.0.rst:312:- Bug in :meth:`DataFrame.xs` when used with :class:`IndexSlice` raises ``TypeError`` with message `Expected label or tuple of labels` (:issue:`35301`)
doc/source/whatsnew/v1.2.0.rst:320:- In :meth:`read_csv` `float_precision='round_trip'` now handles `decimal` and `thousands` parameters (:issue:`35365`)
doc/source/whatsnew/v1.2.0.rst:322:- :meth:`to_csv` passes compression arguments for `'gzip'` always to `gzip.GzipFile` (:issue:`28103`)
doc/source/whatsnew/v1.2.0.rst:324:- :meth:`to_csv` and :meth:`read_csv` did not honor `compression` and `encoding` for path-like objects that are internally converted to file-like objects (:issue:`35677`, :issue:`26124`, and :issue:`32392`)
doc/source/whatsnew/v1.2.0.rst:327:- Bug in :meth:`read_csv` with `engine='python'` truncating data if multiple items present in first row and first element started with BOM (:issue:`36343`)
doc/source/whatsnew/v1.2.0.rst:328:- Removed ``private_key`` and ``verbose`` from :func:`read_gbq` as they are no longer supported in `pandas-gbq` (:issue:`34654` :issue:`30200`)

This is going to fail for now so need to figure out how to make it pass.

[jbrockmendel]

nitpicks, generally looks good

[jreback]

lgtm. pls merge when ready