Add MF-3: Headings and sections

MF-3%3A Headings and sections.-.md

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+This is a simple Markdown example demonstrating heading levels and basic text.
+
+# Main Heading
+This is the first paragraph under the main heading. It's just plain text.
+
+## Sub-Heading
+This is a paragraph for the sub-heading. It contains more simple text.
+
+### Section Heading
+This is a paragraph for a smaller section.
+
+#### Minor Section
+This is a brief paragraph.