Website/Asciidoc: enable TOC generation and footnotes

this resolves a long standing issue; the functionality
was provided by Asciidoc out of the box, relying on the
standard page configuration. Creating our own page layout
unintentionally broke these functions

TOC generation can now optionally be enabled by setting the
page attirbute ":toc:" (in the page header). Footnotes are
available through the "footnote:[text]" macro. Our CSS
is adjusted accordingly to display the TOC in the sidebar
and the Footnotes above the footer