Symbols panels show markdown headers are nested

[lunarwhite]

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Bug

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Take https://github.com/community/community/blob/main/CODE_OF_CONDUCT.md?plain=1 for an example.

Open Symbols panels, we will see every markdown headers are nested, no matter H1/H2/H3.

## Overview and Purpose

## Pledge

## Standards

### _Best Practices for Maintaining a Strong Community_

### _What is not Allowed_

...

It's weird that symbols didn't keep their original structures.