Migrate these pages from Confluence wiki to official wiki: * https://cwiki.apache.org/confluence/display/NUTTX/Special+Files+and+Device+Numbers * https://cwiki.apache.org/confluence/display/NUTTX/Pseudo+File+System * https://cwiki.apache.org/confluence/display/NUTTX/How+NXFFS+Works * https://cwiki.apache.org/confluence/display/NUTTX/NuttX+File+System * https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=139630111
