documentation-builder: Drop as it is now unused

We used to build our docs with hotdoc and that required some special bits.

We're still using Pandoc and LaTeX which are annoyingly special as well, but definitely less unique.

We moved the website hosting our documentation to Hugo a long time ago, which this image does not even ship, and the last usage for generating PDF has been dropped a few months ago: docs/apertis-website!508 (merged)