added docs dir and markdown documentation

docs/john_gruber.md

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+John Gruber is the creator of Markdown.\
+He describes the syntax here:\
+https://daringfireball.net/projects/markdown/syntax
+
+#### headers
+
+https://daringfireball.net/projects/markdown/syntax#header
+
+> Atx-style headers use 1-6 hash characters at the start \
+of the line, corresponding to header levels 1-6.
+
+=> that means header level 6 is the maximum level (vim and bat also do not highlight \
+level 7 headings)
+
+But there are also "Setext-style headers" that can only create \
+level 1 and 2 headers.