fix: load boldsymbol mathjax extension

<!--- Provide a general summary of your changes here. --> ## Checklist <!--- Go over all the following points, and put an `x` in all the boxes that apply. --> <!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! --> - [x] I have read the **CONTRIBUTING** document - [x] I hereby agree to the terms of either individual CLA available at: https://toloka.ai/legal/cla_individual or corporate CLA available at: https://toloka.ai/legal/cla_corporate, whichever is applicable to me, as it pertains to direct contributions to crowd-kit only - [x] My change requires a change to the documentation - [ ] I have updated the documentation accordingly - [ ] I have added tests to cover my changes - [ ] All new and existing tests passed