[Automated] Fix relative figure paths in slides/

[github-actions]

Automated changes by fix-figure-paths.yaml workflow.
Compare and merge to keep figure paths compatible with compilation outside of overleaf.

Explanation:

From what I understand, overleaf automatically tab-completes image paths relative to the project root, e.g.
slides/supervised-regression/figure/nutshell-regression-poly-plot-1.pdf - which is apparently fine on overleaf
but causes errors when trying to compile slides locally, where paths relative to the folder containing the
.tex file are required (i.e. figure/nutshell-regression-poly-plot-1.pdf).

As far as I understand it the latter path should also work on overleaf.
To locally auto-fix this, the following command can be used on Linux:

sed -i -E "s\slides/[0-9a-z-]+/figure\figure\g" $(find slides -iname "*.tex")

Which is exactly what this workflow does.
Note that macOS requires gnu-sed as the default BSD sed behaves differently.