bug(docs): Overview Content of a11y package is random headings

[twerske]

Reproduction

Steps to reproduce:

1. https://material.angular.io/cdk/a11y/overview

Expected Behavior

Overview should show the a11y tools available:
ListKeyManager

• Example

FocusTrap

• Example

InteractivityChecker

• Example
  etc

Actual Behavior

Overview is a mixture of headers from the different a11y tooling. Markdown may be tagged wrong?

Environment

v11 docs site

[jelbourn]

Something is definitely weird with how the TOC is being generated. Might be related to #21751. @Splaktar want to take a look since you dealt with this recently?

[Splaktar]

I will take a look.

[Splaktar]

It looks like this was messed up when more content was added in v6. Here's the v5 site: https://v5.material.angular.io/cdk/a11y/overview.