Only valid HTML is rendered into the generated pages

Only valid HTML is rendered into the generated pages

- The *page title* is rendered inside a `<h1>`-tag at the beginning of a
  generated page. This tag can be easily referenced from CSS and HTML by
  its `id`-attribute *sili-title*, or replaced with the help of `sed` in
  a post-processing script.
- The *page body* is rendered insied a `<div>`-tag with the `id`
  *sili-body*.
- The generated JSON, that captures the project meta-data and the site
  structure, is rendered insiede a `<script>`-tag with the `id`
  *sili-json*.