docs: a layout of logs section of yellow paper (AztecProtocol#3582)

Partially AztecProtocol#3106

**Note**: There is not much in this PR since I don't really know how
much of what [Mike](https://aztecprotocol.slack.com/team/U02B9GDJKAN)
wrote
[here](https://forum.aztec.network/t/issues-with-logs/2609/4?u=jan) is
final (and I don't really understand what is data bus so don't really
have the knowledge to finish that). I pointed out in the PR what needs
to be discussed and expanded upon.

Author: benesjan

yellow-paper/docs/logs/index.md

@@ -0,0 +1,45 @@
+---
+title: Logs
+---
+
+Logs on Aztec are similar to logs on Ethereum and their goal is to allow smart contracts to communicate arbitrary data to the outside world.
+Logs are events which are emitted during contract function execution.
+Aztec protocol gives users the following assurances:
+1. The logs get published,
+2. log integrity (the logs are not modified once emitted),
+3. address of the source contract is verified to be correct (a contract can't impersonate another one).
+
+:::warning Expand on how this is ensured in circuits once [this discussion](https://forum.aztec.network/t/issues-with-logs/2609/) is wrapped up.
+:::
+
+# Types
+There are 2 kinds of logs in Aztec protocol: unencrypted and encrypted.
+
+## Unencrypted
+Unencrypted logs are used to communicate public information out of smart contracts.
+Unencrypted logs can be emitted from both public and private functions.
+
+:::info
+Emitting unencrypted logs from private functions can be a privacy leak but we decided to not forbid it in-protocol because it might allow for interesting usecases like custom encryption schemes using FHE etc.
+:::
+
+## Encrypted
+Encrypted logs can be emitted only from private functions.
+This is because to encrypt the log we need to get a secret and it's impossible to privately manage secrets in public domain.
+
+:::info
+An important usecase of encrypted logs is delivery of notes (note commitment/hash preimage) to recipients.
+:::
+
+### Log encryption
+
+:::warning
+Expand here how exactly the logs are encrypted.
+I (benesjan) am not up-to-date on what is the encryption end-game.
+:::
+
+# Encoding
+Just like on Ethereum, logs are ABI encoded.
+
+:::warning As far as I know the encoding will be happening in app circuit and won't be enforced by protocol. Should this section not be here for this reason?
+:::