Add documentation for IMarkdownToHtmlConverter for CMS 17.2

[AndyButland]

📋 Description

This PR adds documentation for the IMarkdownToHtmlConverter planned for introduction in Umbraco CMS 17.2.

📎 Related Issues (if applicable)

See PR umbraco/Umbraco-CMS#21242 (and related issues umbraco/Umbraco-CMS#21238 and umbraco/Umbraco-CMS#19500).

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

Umbraco CMS 17.2

Deadline (if relevant)

Should be published along with the 17.2 RC due on 5th February.

[sofietoft]

Thanks for the PR @AndyButland

I've added a couple of comments. Check it out when you have some time.