gitlab-ci: Run jekyll to build and package te site.

The website uses the jekyll static site generator to construct
the html tree from mostly markdown source files. Invoke this
inside that project's container image to build and package
the site as an archive.

This both verifies that the code builds without error, and
makes deployment easier since the static files can be downloaded
as an artefact fromg gitlab's artifact archive.
1 job for gitlab-ci
Status Job ID Name Coverage
  Test
canceled #10220
build