[llvm] [github][Security] Add the llvm-security-group team as code owner for Security.rst (PR #106255)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> > I don't think the group is _publicly_ visible -- in a private window, I'm prompted to log in first.
> 
> The same thing happens with other teams, e.g. reviewers-libcxx . I wonder if @kbeyls can see the settings of the group.

The group should be public, see this screenshot from the team settings I just took (not sure in how far others have access to seeing these settings):

<img width="524" alt="Screenshot 2024-08-27 at 21 38 51" src="https://github.com/user-attachments/assets/d9145b4e-75af-4de5-bed8-01abaa3de7fe">


https://github.com/llvm/llvm-project/pull/106255