Docs: reformat custom issue tracker steps

What does this MR do?

The Custom Issue Tracker page is pretty sparse and caused some recent confusion on whether or not the current integration supported other GitLab projects as custom issue trackers, so I wanted to expand on this.

I think ideally we might want to break this out into separate pages for each, but for now I just wanted to clarify which trackers we currently have first-class support for.

Since all integration pages are the same, I suppose they are all essentially the same feature but we market them more specifically so I think the docs should reflect that?

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• 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