Delete "Learn more" link from the Linked items widget

Problem to solve

The Linked items widget in work items is shared between different work item types, like tasks, OKRs, epics, and soon issues. Its Learn more link points to https://docs.gitlab.com/ee/user/okrs.html#linked-items-in-okrs, regardless of the work item type.

It's possible to add conditional logic and show different docs link in different work item types, but we already have a lot of duplicated content between those types.

I propose creating a page called Linked items:

1. MVC 1 (#510355 (closed))

   From the new page, link to:

   • https://docs.gitlab.com/ee/user/project/issues/related_issues.html
   • https://docs.gitlab.com/ee/user/group/epics/linked_epics.html
   • https://docs.gitlab.com/ee/user/tasks.html#linked-items-in-tasks
   • https://docs.gitlab.com/ee/user/okrs.html#linked-items-in-okrs

2. MVC 2 (this issue)

   The Learn more link should probably be removed from the widget altogether. We're trying to avoid link loops where the UI is just linking to UI instructions in the docs. There's a brief explanation of what a link item is, but that info could be included in a popover.

   The link is here https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/work_items/components/work_item_relationships/work_item_relationships.vue#L32.

3. MVC 3 (new issue)

   When epics and issues are fully migrated and the steps are the same, move the main steps to the new page and deprecate the other pages.

Who can address the issue

@msedlakjakubowski with @ramistry

Other links/references

Docs IA: Reorganize the "Plan and track work" s... (&16344 - closed)