fs(3lua): init man pages

Initialise man pages for the fs module.

man/fs/fs.rmdir.3lua.scd

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+FS.RMDIR(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*fs.rmdir* - remove an empty directory
+
+# SYNOPSIS
+*fs.rmdir*(_dir_: *string*)
+
+# DESCRIPTION
+*fs.rmdir* removes the empty directory _dir_.
+
+If you need to recursively remove a directory containing other files
+or directories, you should use fs.remove(3lua) instead.
+
+If an error occurs,
+*fs.rmdir* returns nil, an error message and
+a platform-dependent error code.
+There are many reasons *fs.rmdir* can fail;
+see rmdir(2) for a list of possible error values.
+
+# EXAMPLES
+Remove the empty directory _mydir_:
+
+```
+assert(fs.mkdir("mydir"))
+fs.rmdir("mydir")
+```
+
+# SEE ALSO
+callisto(3lua), fs(3lua), fs.remove(3lua), rmdir(2)