KSelect: Add documentation about the new events and slots

[AlexVelezLl]

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Summary

Document all props, slots, events, and public methods of KSelect that are not documented yet.

Context

In #549 we introduced an important refactor of KSelect as part of moving away from KeenUiSelect vendored component, and as a result we ended up having all the events, slots and public methods exposed by KeenUiSelect in KSelect, and the most of them doesn't have any description in our docs.

Desired behavior

Would be great to have this new API documented, so anyone that reads them can understand it.

Guidance

Note that the props, slots, and events documentation is auto-generated from JSDoc strings in the Vue component - so that's the right place where to document it all. See https://github.com/learningequality/kolibri-design-system/blob/release-v4/dev_docs/04_how_to_update_docs.md#updating-the-library-components-documentation.