Broken tutorial in docs

[BenediktPeterseim]

I noticed that the tutorial "Getting Started - Matrix Multiplication" (https://egglog-python.readthedocs.io/latest/tutorials/getting-started.html) uses a decorator @egraph.class_ that seems to no longer exist, causing an attribute error. I think it should be removed, and the syntax for creating expressions and registering rewrite rules should be adjusted.

[saulshanabrook]

Thank you! Yeah we should also check why the docs builds are passing even with this error and fix that so these are highlighted more.