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

[Tulio Magno Quites Machado Filho via llvm-commits]

tuliom wrote:

Unfortunately this proposal is not going to work, because it requires to give write access to the team. Thanks @tstellar !
He also suggested that we create a [pr-subscribers team](https://llvm.org/docs/DeveloperPolicy.html#stay-informed).  This kind of team is automatically mentioned in the PR. Notice the team is not added as reviewer. But it has the positive side of making write permission more complex by adding yet another team with write permission.

If nobody objects to this new proposal, I will update this PR in the following days.

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