DOC: Fix all flake8 issues and warning in the documentation pages

[datapythonista]

This issue is to track the pending work in making the .rst documentation pages be free of PEP-8 and other flake8 issues, and to not generate warnings.

Page	flake8	warnings
10min.rst	FAIL: #24174	FAIL
advanced.rst	OK: #24182	FAIL
api.rst	OK	FAIL
basics.rst	FAIL	FAIL
categorical.rst	OK: #24182	FAIL
comparison_with_r.rst	OK	FAIL
comparison_with_sas.rst	OK: #24182	FAIL
comparison_with_sql.rst	OK: #24182	FAIL
comparison_with_stata.rst	OK: #24182	FAIL
computation.rst	OK	FAIL
contributing.rst	OK: #24182	FAIL
contributing_docstring.rst	FAIL	FAIL
cookbook.rst	OK: #23794	FAIL
developer.rst	OK	FAIL
dsintro.rst	FAIL: #24175	FAIL
ecosystem.rst	OK	FAIL
enhancingperf.rst	FAIL: #24176	FAIL
extending.rst	OK: #24182	FAIL
gotchas.rst	OK	FAIL
groupby.rst	FAIL: #24178	FAIL
index.rst	OK	FAIL
indexing.rst	PR: #24089	FAIL
install.rst	OK	FAIL
internals.rst	OK	FAIL
io.rst	OK: #23791	FAIL
merging.rst	FAIL: #24179	FAIL
missing_data.rst	PR: #24089	FAIL
options.rst	PR: #24089	FAIL
overview.rst	OK	FAIL
r_interface.rst	OK	FAIL
release.rst	PR: #24089	FAIL
releases.rst	OK	FAIL
reshaping.rst	FAIL: #24180	FAIL
sparse.rst	OK	FAIL
text.rst	OK	FAIL
timedeltas.rst	OK	FAIL
timeseries.rst	OK	FAIL
tutorials.rst	OK	FAIL
visualization.rst	FAIL: #24181	FAIL

After this issue is closed, we shouldn't have any file (other than the whatsnew pages) in the exclude section of flake8-rst in setup.cfg, and we should be able to enable the -W flag for sphinx-build in doc/make.py.

CC: @FHaase

[jbrockmendel]

@datapythonista id like to update this table. is there a command i can run to do the appropriate checks?

[datapythonista]

I think we are already calling sphinx-build with -W, I think we fixed the warnings in the docs a long time ago (#26852 seems to be the PR).

For pep8 in the docs, the command is flake8-rst, which seems to be called without exclusions: https://github.com/pandas-dev/pandas/blob/master/ci/code_checks.sh#L80

So, I guess the whole table should be set to OK, and we can close this issue. I see all the issues in the table are already closed.