[docs] List slots in MUI Base components documentation

[michaldudak]

Problem

When landing on https://mui.com/base/api/select-unstyled/, as a developer you have no ways to know which class names you can target to customize. It was left as feedback in https://mui-org.slack.com/archives/C041SDSF32L/p1673303855354929

Proposed solution

Add a section listing all the available slots to all of the Base components' docs. It would be similar to https://mui.com/x/api/data-grid/data-grid-pro/#slots. It wouldn't provide which class names are available, but since they follow the same convention as the slots, this should already help a lot, maybe killing two birds with one stone.