Please note, that this is an example skin only. That is the reason it is not installed via the extension installer.
/
etc/weewx/skins/Skymap (in case of the package installation, otherwise the skin directory of your installation) is the directory the source files of the skin are stored. In contrary
HTML_ROOT is the directory where WeeWX saves the files it creates. They are completely independent. You can even leave out the definition of
HTML_ROOT in the skin section, which results in creating the file in
/var/www/html/weewx by default.