fs(3lua): init man pages

Initialise man pages for the fs module.

man/fs/fs.mkdir.3lua.scd

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+FS.MKDIR(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*fs.mkdir* - create a directory
+
+# SYNOPSIS
+*fs.mkdir*(_path_: *string*)
+
+# DESCRIPTION
+*fs.mkdir* creates a new directory at _path_.
+
+If you're trying to create multiple directories at once,
+you should use fs.mkpath(3lua) instead.
+
+If an error occurs,
+*fs.mkdir* returns nil, an error message and
+a platform-dependent error code.
+There are many reasons *fs.mkdir* can fail;
+see mkdir(2) for a list of possible error values.
+
+# EXAMPLES
+Create the user's home directory:
+
+	fs.mkdir(environ["HOME"])
+
+# SEE ALSO
+callisto(3lua), fs(3lua), fs.mkpath(3lua), mkdir(2)