"code_of_conduct.rst" and "contributors.rst" both contain RST include
directives, but their file types are symoblic links. This commit turns them
into proper files.
There were typos in symlinks target names resulting in following
warrning on checkout (at least in Mynewt newt tool):
* Warning: stat mynewt/repos/tinyusb/docs/contributing/code_of_conduct.rst: no such file or directory
* Warning: stat mynewt/repos/tinyusb/docs/info/contributors.rst: no such file or directory
