New file to add to security documentation, "How to reset your root password"

doc/security/reset_root_password.md

+# How to reset your root password:
+
+Log into your server with root privileges. Then start a Ruby on Rails console.
+
+Start the console with this command:
+
+```bash
+> gitlab-rails console production
+```
+
+Wait until the console has loaded.
+
+There are multiple ways to find your user. You can search for email or username.
+
+```bash
+irb(main):001:0> u = User.where(id: 1).first
+or
+user = User.find_by(email: 'admin@local.host')
+```
+
+Now you can change your password:
+
+```bash
+> u.password = 'secret_pass'
+> u.password_confirmation = 'secret_pass'
+```
+
+It's important that you change both password and password_confirmation to make it work.
+
+Don't forget to save the changes.
+
+```bash
+> u.save!
+```
+
+Exit the console and try to login with your new password.