fs(3lua): init man pages

Initialise man pages for the fs module.

man/fs/fs.mkpath.3lua.scd

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+FS.MKPATH(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*fs.mkpath* - create a tree of directories
+
+# SYNOPSIS
+*fs.mkpath*(_path_: *string*)
+
+# DESCRIPTION
+*fs.mkpath* creates multiple directories specified by one path.
+For example, _fs.mkpath("/usr/local/share/man/man3lua")_ would first
+check that _/usr_ is a directory, and if not attempt to create it, and
+then repeat that same process for each following component of the path,
+similar to the behaviour of mkdir(1)'s *-p* option.
+
+For creating single directories without this advanced functionality,
+it might better suit you to use fs.mkdir(3lua) instead.
+
+If an error occurs,
+*fs.mkpath* returns nil, an error message and
+a platform-dependent error code.
+There are many reasons *fs.mkpath* can fail;
+see mkdir(2) and stat(2) for possible error values.
+
+# EXAMPLES
+Create the directory _/usr/local/include/callisto_, creating
+intermediate directories as required:
+
+	fs.mkpath("/usr/local/include/callisto")
+
+# SEE ALSO
+callisto(3lua), fs(3lua), fs.mkdir(3lua), mkdir(2)