Implement visual regression tests for documentation examples

🐞 **Problem**

Every time Dependabot opens a pull-request updating some documentation dependency, we are forced to manually inspect the rendering of the results.

We know in advance the changes introduced in our corporate theme but we are clueless about other tools like Jupytext or nbsphinx.

💡 **Idea**

It would be great to implement some visual regression tests in the form of a Sphinx hook. This mechanism could check, at least, for the output images generated when building the gallery of examples, avoiding us to manually check the new results.

Start by locating the Sphinx documentation build and the gallery of examples; no file or existing test path is named in the issue. Review how Dependabot updates affect the corporate theme, Jupytext, and nbsphinx, then determine how a Sphinx hook could compare generated output images. Done means dependency updates can run the visual checks and report changed gallery images without manual inspection.

Written by the indexing model from the issue text.