📝(documentation) document webhooks and matrix interop

Briefly document matrix interop and webhooks basic requirements

Author: mjeammet

docs/interoperability/matrix.md

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+# Interoperability with Matrix API
+
+Matrix is a protocol for secure, decentralized communication. It's the protocol used by [Tchap](https://www.tchap.gouv.fr/), another application of [La Suite](https://lasuite.numerique.gouv.fr/).
+
+## Purpose
+
+With webhooks using this protocol, you can automate the invitation and removal of your colleagues' accounts to Matrix chat rooms.
+
+> [!NOTE]   
+> The first version of this feature requires inviting a bot and giving it moderation permission to your Matrix room. This won't be necessary in future versions of the feature, as invitations/removal will be made in your name using your [ProConnect](https://www.proconnect.gouv.fr/) account.

docs/interoperability/webhooks.md

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+# General considerations on webhooks
+
+Webhooks are small add-ons you can link to your teams to automate the sending of information from People to other apps. Configure your webhooks to point to the tools or ressources you want to share with your group (such as a chatroom, a kanban board or a specific collection in your knowledge base). Whenever you add (or remove) someone from your team in People, access will also be granted (or revoked) to your resources. 
+
+## Requirements for webhook clients
+
+Your webhook client must contain "invite_user_to_group" and "remove_user_from_group" methods, describing how to connect to your ressource and to grant and revoke accesses.