Docs: Merge EE doc/​user/discussions to CE

What does this MR do?

Ported to EE in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11650

This aligns the CE and EE doc/​user/discussions docs, as part of the effort to combine docs into a single codebase (&199 (closed)).

This MR (and the EE Port):

• Brings the EE Merge Request Reviews info into user/discussions/index.md and 10 images to go along with it.
• Adds a single badge to a mention of epics.

When both the EE Port and this MR are merged, the doc/​user/discussions docs should hopefully be aligned between CE and EE.

Then, please merge: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11877, and the issue will be closed.

Related issues

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/60842

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• 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.