Fix html5/document – cleanup header and add some meta tags

To be consistent with the built-in version of the backend (based on [e37eb56](https://github.com/asciidoctor/asciidoctor/commit/e37eb56a0aa5c0dfa82c150caa50afbfad1c29ae#diff-df3b6463915cbe086c0286f7d27617d8), [402273b](https://github.com/asciidoctor/asciidoctor/commit/402273b05f0e48ae4c26a977583c613b6a664f87)).