TOC entry for heading with Mathjax is not rendered correctly

### Is this a hackmd.io issue?

- [X] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem
TOC entry is not rendered correctly when Mathjax is used in the heading.
This issue is the reported previously (#277) two years ago with no response.

#### Current behaviour
TOC entry for heading with Mathjax is rendered with leftover HTML, causing it to be gibberish.

> Screenshot (optional)

Provide steps to reproduce:
1. Create new note
2. Add heading with Mathjax, e.g. `# $\alpha$`
3. Check the TOC sidebar / add `[TOC]`
4. See error

note link: https://hackmd.io/jCBMbG6UQpm-rdRR0pq_4w?both

#### Expected behaviour
Either:
- Mathjax is rendered correctly in TOC entry, or,
- Text version of the heading is used instead in TOC (e.g. using `textContent`, which is already been used in the `title` attribute of the TOC entry)

#### Environment

I use HackMD on:

**Desktop**
- OS: Windows 11
- Browser: firefox
- Browser Version: 116.0.2

#### Additional context
Judging by the code in codimd, the `getHeaderContent()` function in `public/js/extra.js` returns the `innerHTML` of the rendered `HTMLHeadingElement` with simple trimming, which is probably the cause of the problem.