provide draft docs for overall flows from gemini interactive session

[link2xt]

Moved this out of the main branch so we don't have AI slop about "highly specialized path designed to minimize battery drain and maximize privacy" on the readme

[link2xt]

If anyone needs docs on how it works, I just wrote down how push notifications work, can as well be reused:

1. Delta Chat on Android obtains FCM push token and gives it to the Rust core library: https://github.com/deltachat/deltachat-android/blob/c45a47e53c889016b7ddcf0d5f2dc5cc7f4b4090/src/gplay/java/org/thoughtcrime/securesms/notifications/FcmReceiveService.java#L64-L65
2. Push token is given to the Rust core library via dc_accounts_set_push_device_token:
   https://github.com/chatmail/core/blob/f0a12d493c0233baf31778ac6c8b0cf4e709d5cc/deltachat-ffi/deltachat.h#L3349-L3356
3. Inside the IMAP loop push token is encrypted to the hardcoded public OpenPGP key: https://github.com/chatmail/core/blob/f0a12d493c0233baf31778ac6c8b0cf4e709d5cc/src/imap.rs#L1670-L1671
   Private key belongs to central gateway https://notifications.delta.chat/
4. Encrypted token is given to the IMAP server via SETMETADATA "INBOX" (/private/devicetoken <encrypted-token-goes-here>) command.
5. Token is received by IMAP METADATA handler of a chatmail relay (dovecot with custom METADATA handler) here:
   https://github.com/chatmail/relay/blob/65b660c41334c358027fe46989dd7797bed8126c/chatmaild/src/chatmaild/metadata.py#L116-L118
6. chatmail relay gives remembered token to https://notifications.delta.chat/notify via HTTP POST request:
   https://github.com/chatmail/relay/blob/65b660c41334c358027fe46989dd7797bed8126c/chatmaild/src/chatmaild/notifier.py#L160
7. The token is decrypted by notifications.delta.chat service and is then sent to FCM or APNS or UBPorts (ubuntu touch) API here:
   

   notifiers/src/server.rs

   Lines 274 to 297 in 1abd85c

   	/// Notifies a single device with a visible notification.
   	async fn notify_device(
   	axum::extract::State(state): axum::extract::State<State>,
   	mut device_token: String,
   	) -> Result<StatusCode, AppError> {
   	// Decrypt the token if it is OpenPGP-encrypted.
   	if let Some(openpgp_device_token) = device_token.strip_prefix("openpgp:") {
   	match state.openpgp_decryptor().decrypt(openpgp_device_token) {
   	Ok(decrypted_device_token) => {
   	device_token = decrypted_device_token;
   	}
   	Err(err) => {
   	error!("Failed to decrypt device token: {:#}.", err);
   	let metrics = state.metrics();
   	metrics.openpgp_decryption_failures_total.inc();
   	// Return 410 Gone response so email server can remove the token.
   	return Ok(StatusCode::GONE);
   	}
   	}
   	}
   	info!("Got direct notification for {device_token}.");