chore : Update Documentation for instagram test app in self hosted

[sojan-official]

I would like to inform you about the deprecated documentation on how to configure an Instagram test app. The link https://www.chatwoot.com/docs/self-hosted/configuration/features/integrations/instagram-channel-setup/ is no longer valid, as the Instagram Basic Display product is not available in test apps anymore. Now, it is necessary to add a test user in Business Meta, and this user must accept the invitation to test sending and receiving Instagram messages.

Could you please forward this information to the responsible team to update the documentation?

ref: https://app.chatwoot.com/app/accounts/1/conversations/32893