docs: add filenames to `Marking text` examples

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[noam-honig]

@AriPerkkio not exactly, I see my mistake (sorry, I was just editing on github).

The original image confused me - it made me thing the console log should appear twice - I thought it were two examples that are not written correctly.

I fixed it now to be one single example, and also to use title to explain that one is md and the other is code - like other examples in the document.

I hope thats ok

[AriPerkkio]

This is now not aligned with the other example 🤔

https://github.com/stackblitz/tutorialkit/blob/8547af7665439aeaf20c4cb5722a3a04fd6f7cf9/docs/tutorialkit.dev/src/content/docs/guides/creating-content.mdx?plain=1#L175-L193

[noam-honig]

Sorry - long day and lots of content switching - I hope its ok now