Validate that data table cells have a header with axe-core

Context

The Dutch Web Guidelines (DWG) validation is being removed. Its rule 11s2 checks that data tables have th cells, which is required by WCAG 1.3.1 (level A). axe-core has an equivalent rule, td-has-header, that is more accurate: it only checks data cells of tables larger than 3x3 and ignores layout tables. Because the rule is tagged experimental, it is not run by default, so nothing covers this requirement once DWG is gone.

Proposal

• Enable td-has-header explicitly in WCAGContext, as a warning first.
• Make it fail the build once the reported violations have been checked for false positives.

Raised in https://forum.xwiki.org/t/remove-the-dutch-web-guidelines-validation/19056