Added documentation for goal attribution

[AltamashShaikh]

Description:

Added documentation for goal attribution Fixes: #PG-2659.

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

[AltamashShaikh]

The code looks good and I was able to view the documentation for all of the sections in the Campaigns view. I didn't see documentation in all of the places the screenshots show in the Jira issue, but I'm guessing those required PRs in other repos.

Yes for detail review, you can check this core PR