Optionally use H2 as the base level for hierarchy for the ToC

Changes introduced in the context of https://jira.xwiki.org/browse/XWIKI-21370 and https://forum.xwiki.org/t/accessibility-proposal-removing-extra-h1s-from-the-page/13560/ make it so that the best practice is to have page content only contain H2 and below headings (because all that hierarchy is below the page title, a H1).

This would cause the ToC macro to 'start' at the second level instead of the first one.

I think it would make sense to remove the first level of heading if it's empty.