docs: add migration guide and notes for upcoming react 18 upgrade (#66656)
* add information to what's new about upcoming react 18 upgrade
* update structure
* Apply suggestions from code review
Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>
* add docs to plugin migration guide
* add link to forum discussion
* apply suggestions from review
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Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>
(cherry picked from commit 1917857186)
Co-authored-by: Ashley Harrison <ashley.harrison@grafana.com>