Overview in Markdown files

[scriptcoded]

Issue Type: Feature Request

While reading the latest release notes within VS Code, I realized that there's one feature I lacked, the ability to easily navigate the Markdown file.

If you've ever read a long Google Docs document you might have a clue as to what I'm talking about. On the righthand side of the screen, theres a navigation pane, giving a clear overview of the document, showing all headings and subheadings withing the document. The same should be present within Markdown files within VS Code.

I put together a quick screenshot for reference.

VS Code version: Code 1.22.2 (3aeede7, 2018-04-12T16:38:45.278Z)
OS version: Windows_NT x64 10.0.16299

[mjbvz]

Duplicate of #5605 . Go to symbol already presents an outline and can be used to quickly jump between sections