docs(integrations): Add steps for setting up a video chat provider.
docs(integrations): Add steps for setting up a video chat provider.

This is a preparatory commit that documents
steps for setting a video chat provider for an
organization. It is added as a separate section
in /help/start-a-call. Permalinks to this section
are utilized in succeeding commits which move
documentation of our video call providers to
`/integrations`, as asked in issue #17588.

Tweaked by tabbott for clearer/more consist language.

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