Exclude docs from ee_compat_check

What does this MR do?

Now that the docs is using a single codebase, and all docs submissions must be submitted in CE (and are rejected in EE), the ee_compat_check CI test no longer needs to run on docs-only changes.

In addition, the test can ignore docs completely, in regular branches, as docs changed submitted to EE will be picked up by ee_specific_lines_check, and can be submitted freely in any CE branch.

Ported to EE in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14966

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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