[docs] Fix API page regression: bring back slots section

[alexfauquette]

Fixes #9857

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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Updated pages

No updates.

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	318.7	490.6	423.1	406.14	72.539
Sort 100k rows	ms	481.2	898.4	827.9	737.48	167.675
Select 100k rows	ms	181.2	220.5	198.3	199.12	12.609
Deselect 100k rows	ms	108.4	258.8	214.9	190.18	58.851

Generated by 🚫 dangerJS against 971d042

[LukasTy]

I'm not sure if the current form of providing the default is clear in any way... 🤔
IMHO, I would struggle to understand what that grey text means... 🤷
Secondly, I'm pretty sure that none of the slots are required. 🤔

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[alexfauquette]

For the Required it will be fixed by mui/material-ui#38183

About removing the description, it might interfere with the docs of other packages (joy also has a class name section). I keep it as it is for now, since bigger modification might impact that aspect