HTML indexes, but not deep linking

[tiltos]

Overall, this is a great plugin.
I've got a bit of an issue though. It appears that search can index html fine (including that in mdx files), however it cannot deep-link to it.

For example, ``/docs/hello` :

## Parent heading

content here

<h3>example</h3>

The expected behaviour result in this example is to link to the parent heading (/docs/hello#parent-heading). This is how normal markdown is linked, as IDs (anchor links) are generated by docusaurus.

The current behaviour is the result links to /docs/hello.

Ideally, if I attach an id to the html heading, eg <h3 id="example">example</h3>, then the search would be able to index that ID and link directly to it (/docs/hello#example).

I realise that headings are treated slightly differently, but basically I want the index to populate the h if it exists on the element. Here's what currently happens in the index:

      {
        "i": 2236,
        "t": "example",
        "u": "/docs/hello/",
        "h": "",
        "p": 2222
      },
      {
        "i": 2240,
        "t": "Parent heading",
        "u": "/docs/hello",
        "h": "#parent-heading",
        "p": 2238
      },