docs(concepts): added docs for general concepts

docs/concepts.md

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+# ICRC - Concepts
+
+## Client
+
+A user or bot. Or to be more precise: the program something uses to communicate with the icrc server.
+
+## Relay
+
+A relay is like a group chat. Clients that are part of a relay have the key to it. The server proofs if a client has
+access to a relay by hashing the clients key to that relay and comparing it with a saved hash.
+
+### User index
+
+Every relay has their own user index, which has the following columns:
+
+- **userid_relay**: The id that other users use to identify a user in this relay.
+- **userid_main**: The users "real" id. (The id they use to authenticate.)
+- **name**: The users nickname in this relay.
+- **public_key**: The public key to exchange symmetric encryption keys with that user.
+