Base: Write some initial man pages

It ain't much, but it's honest work!

Base/usr/share/man/man2/mkdir.md

@@ -0,0 +1,20 @@
+## Name
+
+mkdir - create a directory
+
+## Synopsis
+
+```**c++
+#include <sys/stat.h>
+
+int mkdir(const char* path, mode_t mode);
+```
+
+## Description
+
+Create a new empty directory at the given *path* using the given *mode*.
+
+## Return value
+
+If the directory was created successfully, `mkdir()` returns 0. Otherwise,
+it returns -1 and sets `errno` to describe the error.