[DOCS] Broken mathematical formula for the M-MSR aggregator

### Problem description ## Context The mathematical formula for the M-MSR aggregator (link to the doc page [here)](https://crowd-kit.readthedocs.io/en/stable/classification/) is not rendered correctly: namely, MathJax doesn't render the `\boldsymbol` ## Succes Criteria - [x] \boldsymbol is rendered correctly ### Documentation links ### Visual proof <img src="https://github.com/user-attachments/assets/798eb1e1-f93d-497a-bedd-2cb88490137b" width="400"> (link to the doc page [here)](https://crowd-kit.readthedocs.io/en/stable/classification/) ### Potential fix suggestion - Load the boldsymbol MathJax extension, as [suggested in its docs](https://docs.mathjax.org/en/latest/input/tex/extensions/boldsymbol.html).