docs(vrl): Add `unflatten` vrl function documentation (#21142)

* docs(vrl): Add unflatten documentation

* chore: update spelling checklist

* Update unflatten.cue

Co-authored-by: Esther Kim <esthercodes2019@gmail.com>

* Update flatten.cue

* docs: add new example

* docs: fix spelling

* cue fmt

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

* whitespace

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

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Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>
Co-authored-by: Esther Kim <esthercodes2019@gmail.com>
Co-authored-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>