Preserve markup of headings in TOC sidebar

[septatrix]

Body

Summary

A brief, one or two sentence explanation of the proposal.

Background & Motivation

Include any useful background detail that that explains why this RFC is important.
What are the problems that this RFC sets out to solve? Why now? Be brief!

It can be useful to illustrate your RFC as a user problem in this section.
(ex: "Users have reported that it is difficult to do X in Astro today.")

Goals

• allow automatic generation of sidebars which preserve the markup of the original content (e.g. usage of italic, bold, and strikethrough text in headings - also stuff like badges etc)

Example:

<!-- The bold and italic should also show up in the sidebar -->
# **This** is a _really_ section

Example

Possible API change would be to expand getHeadings() to not only include .text but also .rawHtml

<ul>
/* simplified example ignoring nesting of headings... */
{ getHeadings().map(h => (
    <li set:html={h.rawHtml} />
)) }
</ul>

[septatrix]

Based on withastro/starlight#2527