Add functionality to generate simplified HTML for PDF generation

Apertis currently has a large number of concept documents on
designs.apertis.org. To improve branding, searchability and reduce
maintenance, it would be better to have all these documents under the
main website. One feature that the website is currently missing that is
provided on designs.a.o is PDF generation, which is used when reviewing
concepts with Bosch.

Carry over the PDF generation functionality from designs.a.o and
implement a mechanism for generating PDFs from the Hugo site on
www.apertis.org so that we can move documents over from designs.a.o
without loosing needed functionality.

To do this we will generate simplfied HTML documents, with the
extension `.pdf-in` when documents require this to be generated in
their frontmatter, for example to generate html and the simplified HTML
add:

    outputs = ["html", "pdf-in"]

The create-pdf script will be run for each of these via the CI pipeline
which will use a lightly modified version of the tooling used for
designs.a.o to create the PDF and place it back into the site.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>