Provide original markdown title in TOC

[ccjmne]

Provide original markdown title in TOC.

This commit introduces the "title_raw" property to the Heading struct,
so that the user may get adequately formatted entries in their table of
contents when the original title contains, for example, code highlight
content.

For example:

### The `<summary>` in the `<details>`

Would offer, in heading.title:

The <summary> in the <details>

And in the newly introduced heading.title_raw:

The <code>&lt;summary&gt;</code> in the <code>&lt;details&gt;</code>

Resolves #3023.

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Sanity check:

• Not already addressed by another ongoing PR
• PR made against next?
• Documentation updated?