6.0.2: sphinx 4.0.3 warnings

```console
+ /usr/bin/python3 setup.py build_sphinx -b man --build-dir build/sphinx
running build_sphinx
Running Sphinx v4.0.3
Initializing GitHub plugin
making output directory... done
myst v0.15.1: MdParserConfig(renderer='sphinx', commonmark_only=False, enable_extensions=['dollarmath'], dmath_allow_labels=True, dmath_allow_space=True, dmath_allow_digits=True, dmath_double_inline=False, update_mathjax=True, mathjax_classes='tex2jax_process|mathjax_process|math|output_area', disable_syntax=[], url_schemes=['http', 'https', 'mailto', 'ftp'], heading_anchors=None, heading_slug_func=None, html_meta=[], footnote_transition=True, substitutions=[], sub_delimiters=['{', '}'], words_per_minute=200)
loading intersphinx inventory from https://docs.python.org/3/objects.inv...
loading intersphinx inventory from https://ipython.readthedocs.io/en/latest/objects.inv...
loading intersphinx inventory from https://jupyter.readthedocs.io/en/latest/objects.inv...
building [mo]: targets for 0 po files that are out of date
building [man]: all manpages
updating environment: [new config] 1 added, 0 changed, 0 removed
reading sources... [100%] index
/home/tkloczko/rpmbuild/BUILD/ipykernel-6.0.2/docs/index.rst:11: WARNING: toctree contains reference to nonexisting document 'changelog'
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
writing... python-ipykernel.3 { } done
build succeeded, 1 warning.
```