Set up Security docs in left menubar

We have a whole series of Security-related docs in https://docs.gitlab.com/ee/security/README.html#security, ref https://gitlab.com/gitlab-org/gitlab/-/tree/master/doc/security

We should expose this info in our docs.gitlab.com/ee/ left-hand menubar. Since I see ~15 different subdocs in this area, this is not a trivial task, so I'm setting up this issue.