Add MF-4: Bold italic strikethrough

MF-4%3A Bold italic strikethrough.-.md

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+Here's a Markdown example that uses only visual text formatting features like bold, italic, and strikethrough.
+
+**This is a sentence written in bold.**
+
+_This sentence is written in italics._
+
+***
+
+_**This sentence is both bold and italicized.**_
+
+~~This sentence has a strikethrough on it.~~
+
+This sentence is a mix of plain text, **bold text**, and *italic text*.