docs(vue-testing-library): add slot usage examples

[time1043]

Motivation

Add documentation for the slots option in Vue Testing Library's render function, so users know how to provide slot content when rendering components.

Changes

• Added slots option description under render options.
• Explained keys and values for slots, with a short example.
• Linked to Vue Test Utils slots API for reference.

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[timdeschryver]

@all-contributors please add @time1043 for docs

[allcontributors[bot]]

@timdeschryver

I've put up a pull request to add @time1043! :tada: