Sunset Reactr docs

[LauraLangdon]

With the transition to DeltaV, we'll no longer have users interact with the features of Reactr and can sunset its docs. Of the four Reactr pages, only the language support and Reactr ❤️ Wasm pages are currently referred to outside of the Reactr category. It's not yet obvious whether the language support page should be moved to Compute or to DeltaV, nor whether the Reactr ❤️ Wasm page (referred to on Subo -> Usage) should be absorbed into another category or be deleted.

[LauraLangdon]

Absorbing into #171