Document `get_chat_securejoin_qr_code()`, `check_qr()`, `set_config_from_qr()` (particularly, QR code withdrawal)

Hello,

I am building automation around Delta Chat broadcast channels using the core library and/or RPC server, and I need to **reset/revoke SecureJoin invite links programmatically**.

In the official Delta Chat clients (deltachat android application), there is a “**Reset QR Code**” button in channel settings. When this is used:

* a new SecureJoin QR/link is generated
* previous invite links stop working

However, when using the core programmatically, I can only find the method:

`get_chat_securejoin_qr_code(account_id, chat_id)`

This generates a the same QR code every time and **does not revoke previously generated invites**.

# Questions

1. Is there an RPC method that exposes this functionality?
2. If not, would it be possible to expose such a method in deltachat-rpc-server?

# Use Case

I am implementing automated invite management for channels, I want to reset SecureJoin QR/Links automatically so that publicly exposed links don't work anymore. I need to:

* rotate invite links
* revoke previously shared links
* generate a new SecureJoin QR

# Expected API (example)

Something like:
```
resetSecurejoinQr(chat_id)
```
or
```
rotateSecurejoinToken(chat_id)
```
which would invalidate all previously generated invite links.

Thanks for your work on Delta Chat and the core library!