Convert table header row to contain at least three hyphens

Right now our MD table headers sometimes consist out of single hyphens within a table column. That is not compliant with most of the table MD extensions. E.g. Github explicitly states in https://help.github.com/articles/organizing-information-with-tables/

There must be at least three hyphens in each column of the header row.

Also most of the parsers only detect table headers correctly if they consist of at least three hyphens. This is also the case for the flexmark parser being used in newer JBake versions (https://issues.apache.org/jira/browse/SLING-7776?focusedCommentId=16542694&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16542694).

So all those invalid header rows should be converted to valid ones.