docs: contact-groups rfc (#2775)

docs/rfcs/2023-07-25-contact-groups.md

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+# Contact groups
+
+## Problem
+
+Bad UX when communicating from multiple devices:
+- Lack of data synchronization between devices.
+- Participating in the same conversation from multiple devices requires manually adding each device to a group.
+- For a direct (1-to-1) conversation, it has to be initially created as a group - a contact does not support adding devices to conversation. Not obvious use of groups.
+
+## Solution
+
+Full device synchronization is a complex feature and will not be discussed in this rfc. Instead a proposed solution is to support adding devices to a 1-to-1 conversation, using existing group technology.
+
+Pros:
+- Fully decentralized, no leading and following devices.
+- No complex full state synchronization.
+- More obvious UX than using groups for 1-to-1 conversations.
+- Can be automated to include user devices into all \[new\] conversations.
+
+Cons:
+- Same downsides groups have - increased traffic, diverging group state, possible failures to establish connections (requires out-of-band "connection doctor"?).
+- Muddies existing abstracts of contacts and groups. Adds new orthogonal complexity to many existing features - preferences, incognito, multi-profile, calls, etc.
+- Requires cooperation of contact's client - to support new protocol message for contact-to-group upgrade / contact-group invitation link.
+- Still not full data synchronization. Devices have different profiles and likely different lists of conversations.
+
+### Possible approaches
+
+1. Add device to contact conversation - upgrade contact to group. Optionally automate adding user devices to other conversations.
+2. Managed list of user devices - non optionally automate adding to other conversations.
+
+#### Upgrade contact to group
+
+Pros:
+- Allows to migrate existing contacts to multi-device conversation.
+
+Cons:
+- Button would be hidden in chat info.
+- Requires upgrading contact to group on both sides of the conversation:
+  - Replace contact with group, update entities - chat items, files, etc. How will files in progress migrate?
+  - New protocol message for contact to perform same upgrade + introduction of new device.
+- Implicit list of "my devices", unclear whether to automate adding devices for other contacts? Not as useful if it's not automated?
+
+#### Managed list of user devices
+
+Pros:
+- New button in Settings - easier to discover than chat info button.
+- User devices are automatically added to all new conversations.
+- Start each conversation as a group? -> Doesn't require upgrading contact to group.
+  - New invitation link type - contact-group.
+  - Expanded info in confirmation (when invitation link is for regular contact but joining client has other devices).
+- Can automatically add own devices to existing groups.
+- Doesn't require tricky contact to group upgrade.
+
+Cons:
+- Cannot migrate existing contacts. Create all contacts going forward as contact-groups?