This PR replaces references to _addons_ in the docs with references to
_integrations_.
In doing so, I have also:
- moved `/reference/addons` documents into `/reference/integrations`
- combined the previous "Addons" and "Integrations" categories into a
new "Integrations" category
- added redirects from all the old addons pages to their new locations
I have updated the wording, but have not changed things such as API
paths, event names, etc, because these will not change at the moment
(maybe a breaking change to schedule for v6?).
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Co-authored-by: Tymoteusz Czech <2625371+Tymek@users.noreply.github.com>
