Move Issues docs one level up in the docs side navigation

Problem to solve

The Burndown charts doc is not linked in the left-hand side nav, limiting user discoverability of this page.

However, with the current docs page setup it's only possible to have three levels of navigation.

Further details

Related: Evan's comment in !910 (comment 348616960)

If you want to resolve the problem of the Burndown charts item not appearing in the nav (...) you could promote the whole Issues structure to be top level.

At least as an MVC, which all these extra items and with how integral Issues are to the product, you can argue that even through Issues are scoped to projects, that they could stand alone.

That way, Burndown charts is that the third level, and there's room for possible future expansion.

Proposal

Move Issues one level up, so that Milestones can have subpages and add a link to the Burndown charts doc and, in future, the planned Burnup charts doc.

The global nav top pages could then be:

  • GitLab Docs
  • Install
  • Analytics
  • Clusters
  • Groups
  • Projects
  • Issues
  • Operations
  • CI/CD
  • Packages
  • API
  • User
  • Administrator
  • Security
  • Contributor
  • Customer

Who can address the issue

Mike Lewis, Staff Technical Writer + the rest of @gl-docsteam

Other links/references