Dash support

[lazka]

• https://kapeli.com/docsets#tableofcontents
• https://pypi.org/project/sphinxcontrib-dashbuilder/#description
• https://doc2dash.readthedocs.io/en/stable/#

Ideally we would build the index from the devhelp one as I don't want to add another build type to the pipeline.

@scontini76