Updated Epic docs

What does this MR do?

Updates Epic docs:

• Adds mention and screenshot of Epics and Issues tree.
• Updates screenshots.
• Restructures and reorders some content so that like topics are together.

Related issues

This MR is intended to meet the docs needs of the following issues:

https://gitlab.com/gitlab-org/gitlab-foss/issues/9367 https://gitlab.com/gitlab-org/gitlab-foss/issues/10795

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [ ] If applicable, update the permissions table.
• [ ] Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.