Debian packages should not install files under /var/www. This is not one of the /var directories in the File Hierarchy Standard and is under the control of the local administrator. Packages should not assume that it is the document root for a web server; it is very common for users to change the default document root and packages should not assume that users will keep any particular setting.
Packages that want to make files available via an installed web server should instead put instructions for the local administrator in a README.Debian file and ideally include configuration fragments for common web servers such as Apache.
As an exception, packages are permitted to create the /var/www directory due to its past history as the default document root, but should at most copy over a default file in postinst for a new install. In this case, please add a Lintian override.