No security scan for docs branches

What does this MR do?

As @axil pointed out in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29780#note_185248917, the recent change in the security checks job has the side effect to run them on "docs" branches. This MR not only fixes this and restore the original behavior but also clean-up our definitions to avoid the same mistake in the future. It also leverages the support of multiple extends.

This MR is the continuity of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30081 which had a wrong branch name (and we can't change the branch in an MR).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team