I just ran across this posting and though it's older, I thought I would add my two cents. While sylouuu is correct about placing a namespace directly after the your intended page-level DocBlock, I believe that it is now required to place the '@package' tag in the DocBlock. There is some caution here when dealing with logical versus hierarchical 'packages'. This behavior is basically dependent upon which version of PHPDocumentor you are using.
From: http://manual.phpdoc.org/HTMLSmartyConverter/HandS/phpDocumentor/tutorial_elements.pkg.html
"In phpDocumentor version 1.2.2, a Page-level DocBlock is the first DocBlock in a file if it contains a @package tag."
And at https://github.com/phpDocumentor/phpDocumentor2/issues/910 it can be read that as recently as a month ago, the issue of page-level DocBlocks in files with no PHP code/nodes is still being worked. Thus, as sylouuu stated about adding a namespace, once you do this, you have an documentable element and your problem should be resolved. Additionally, since most likely no documentation for the page will be created without having a node present, this allows your html-only file to have its documentation created.
Hope it helps!
dw