Update link to CI/CD variables page
I noticed these pipeline failures https://gitlab.com/gitlab-org/gitlab-docs/-/pipelines/346507304/failures in Slack ((https://gitlab.slack.com/archives/C011D3TA610/p1627912532000700)) and realized we have a busted link:
[ ERROR ] internal_links - broken reference to file:///ee/security/cicd_variables.html
public/ee/user/project/integrations/mattermost.html:
[ ERROR ] internal_links - broken reference to file:///ee/security/cicd_variables.html
public/ee/user/project/integrations/mattermost_slash_commands.html:
[ ERROR ] internal_links - broken reference to file:///ee/security/cicd_variables.html
public/ee/user/project/integrations/microsoft_teams.html:
[ ERROR ] internal_links - broken reference to file:///ee/security/cicd_variables.html
When I load up that URL, it redirects to https://docs.gitlab.com/ee/ci/variables/index.html#cicd-variable-security, so I'm putting that in as a temp replacement to get pipelines working again. @marcel.amirault will want to take a look for the long-term fix, since this page is devopsverify grouppipeline authoring