Updated docs and example to use @chakra-ui/gatsby-plugin

Editor assessment

The subject states the migration: docs and examples were updated to use the @chakra-ui/gatsby-plugin. Both the artifacts and the target package are named, so the reader knows exactly what was switched and to what. The fix type is defensible for aligning documentation with the current package, though the superseded approach is unstated. No scope is present, though the docs area is identified. There is no body, acceptable for a straightforward alignment change. No footers appear. The past-tense updated is a minor mood note.