Update group integrations sidebar navigation and add docs link

What does this MR do?

• Move Group > Settings > Integrations sub-menu to be below General to match the project-level UI.
• Add "Learn more" links to documentation in group / instance integration listing pages and in the override dropdown on the project integration page.
• Fix Cancel button path on group / instance integration form pages.
• Add some new specs for OverrideDropdown.vue component.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry - Not needed since feature is behind feature flag
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Closes #238983 (closed)