Use "handler" terminology consistently (#10627)

* Use "handler" terminology consistently

* Migrate all handlers to their own section within Runtime APIs

* Fix redirects

* Clean up scheduled() handler docs

* Add redirect for event listener docs

* Update title of migration page

* Update links

* Fix more links

* Clarify request context docs

* Fix remaining broken links

* Fix last link

* Apply suggestions from code review

Co-authored-by: Kate Tungusova <70746074+deadlypants1973@users.noreply.github.com>

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Co-authored-by: Kate Tungusova <70746074+deadlypants1973@users.noreply.github.com>