[Addon Docs] [MDX] Anchors for headings are broken in Storybook 6.4.3

[Celentia]

Describe the bug
MDX anchors in markdown for headings aren't scrolling to the relevant section in Storybook 6.4.3.

To Reproduce

1. Create new or open existing .mdx story;
2. Add an anchor for heading to some docs section, for example:

## Table of Contents
   - [Overview](#overview)
   ...

## Overview

3. Click on "Overview" in the Table of Contents section.

Expected result:
Docs page should scroll to Overview section.

Actual result:
The blank page is shown.

Additional context
This issue is reproducible in Storybook 6.4.3 and Storybook 6.4.0-beta.20. In Storybook 6.3 it has worked fine.

[shilman]

closing as dupe to #16918