Markdown support in TextDocument

[emilk]

What

    rec.log(
        "markdown",
        &TextDocument::new(
            "# Hello\n\
             Markdown with `code`!\n\
             \n\
             A random image:\n\
             \n\
             ![A random image](https://picsum.photos/640/480)",
        )
        .with_media_type(MediaType::markdown()),
    )?;

This will let us add nice documentation explaining our examples.

Not yet supported

• Syntax highlighting of code blocks
  • egui_commomark uses syntect which is too big of a dependency imho, see Consider switching to the syntax highlighting library in egui_extras lampsitter/egui_commonmark#13
• Embedding references to images in the data store
  • We should support something like ![](entity://my/image/entity) where my/image/entity is the entity path to an image you have logged.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

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