gitlab-ci: Run jekyll to build and package the 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
from gitlab's artifact archive.

Signed-off-by: Marvin Scholz <epirat07@gmail.com>
4 jobs for master in 11 minutes and 30 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #10259
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00:11:30

canceled #10258
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build

00:00:22

failed #10257
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build

00:00:09

failed #10256
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build

00:00:09