Add note about secret backend in security.rst

[mik-laj]

Hello,

A small note would be useful. Now we only have information about Fernet on this page.

Are you wondering how to start contributing to this project? Start by reading our contributor guide

Best regards,
Kamil

CC: @kaxil

[mik-laj]

@kaxil Are you planning to continue working on the ticket or can another person start working on it? This is a "good-first-issue", so I'd be happy if new contributors could work on it.

[kaxil]

Un-assigned myself, will let the new contributors work on this one

[mik-laj]

Done via: #10352