Remove extra newlines in contour(f) docs.

[QuLogic]

PR Summary

Followup to #18708.

PR Checklist

• [n/a] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [n/a] New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[timhoffm]

Are you sure this will render correctly. The concatenated docstring must look like:

        Call signature::

            contour([X, Y,] Z, [levels], **kwargs)

        `.contour` and `.contourf` draw ...

Without the empty line after the literal code block, ReST parsing will be messed up. One could likely remove one of the extra lines, but I was too lazy to check.

Anyway CI doc build will show if this works.

[QuLogic]

There's a newline between the end of the signature and """ and a newline between """ and the remaining description. This makes the required 2 lines between the signature and description.

[timhoffm]

Anybody can merge after checking that the docs render correctly.