Add Planning Hierarchy to the docs

doc/topics/plan_and_track.md

@@ -42,5 +42,6 @@ Align your work across teams.
 - [Epics](../user/group/epics/index.md)
   - [Multi-level epics](../user/group/epics/manage_epics.md#multi-level-child-epics)
   - [Epic boards](../user/group/epics/epic_boards.md)
-  - [View heath status](../user/project/issues/managing_issues.md#health-status)
+  - [View health status](../user/project/issues/managing_issues.md#health-status)
 - [Roadmaps](../user/group/roadmap/index.md)
+- [Planning hierarchies](../user/group/planning_hierarchy/index.md)

doc/user/group/epics/index.md

@@ -41,6 +41,8 @@ graph TD
     Child_epic --> Issue2
 ```
 
+Also, read more about possible [planning hierarchies](../planning_hierarchy/index.md).
+
 ## Roadmap in epics **(ULTIMATE)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/7327) in GitLab 11.10.

doc/user/group/planning_hierarchy/index.md

+---
+type: reference
+stage: Plan
+group: Product Planning
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Planning hierarchies **(PREMIUM)**
+
+Planning hierarchies are an integral part of breaking down your work in GitLab.
+To understand how you can use epics and issues together in hierarchies, remember the following:
+
+- [Epics](../epics/index.md) exist in groups.
+- [Issues](../../project/issues/index.md) exist in projects.
+
+GitLab is not opinionated on how you structure your work and the hierarchy you can build with multi-level
+epics. For example, you can use the hierarchy as a folder of issues for bigger initiatives.
+
+To learn about hierarchies in general, common frameworks, and using GitLab for
+portfolio management, see
+[How to use GitLab for Agile portfolio planning and project management](https://about.gitlab.com/blog/2020/11/11/gitlab-for-agile-portfolio-planning-project-management/).
+
+## Hierarchies with epics
+
+With epics, you can achieve the following hierarchy:
+
+```mermaid
+graph TD
+    Group_epic --> Project1_Issue1
+    Group_epic --> Project1_Issue2
+    Group_epic --> Project2_Issue1
+```
+
+### Hierarchies with multi-level epics **(ULTIMATE)**
+
+With the addition of [multi-level epics](../epics/manage_epics.md#multi-level-child-epics) and up to
+seven levels of nested epics, you can achieve the following hierarchy:
+
+```mermaid
+classDiagram
+  direction TD
+  class Epic
+  class Issue
+
+  Epic *-- "0..7" Epic
+  Epic "1" *-- "0..*" Issue
+```
+
+## View ancestry of an epic
+
+In an epic, you can view the ancestors as parents in the right sidebar under **Ancestors**.
+
+![epics state dropdown](img/epic-view-ancestors-in-sidebar_v14_6.png)
+
+## View ancestry of an issue
+
+In an issue, you can view the parented epic above the issue in the right sidebar under **Epic**.
+
+![epics state dropdown](img/issue-view-parent-epic-in-sidebar_v14_6.png)