Fix missing underscore

[butterunderflow]

The current behavior is: underscore is included in the heading, but missing in the sidebar.
For example:

the tail _ of test_ is missing in the sidebar.

It seems was originally raised from here:
https://github.com/rescript-association/rescript-lang.org/blob/b0da61146a0718337957e79569b5386220d55f93/scripts/extract-indices.mjs#L31
We only fetch the value of the first child of a mdast node as the name attribute.
But when a heading contains escape characters like \_, children will be a list. Like

[
  {
    type: 'text',
    value: 'test',
    position: Position { start: [Object], end: [Object], indent: [] }
  },
  {
    type: 'text',
    value: '_',
    position: Position { start: [Object], end: [Object], indent: [] }
  }
]

This PR seems to give a fix.

[ryyppy]

oh, how did we miss that. Looking good, thanks