[web] Hugo build-dir based

Lars Bilkerequested to merge
bilke/ogs:hugo-build-dir into master

Hugo web site generation is now build-directory based. New make-targets for easy web site generation and previewing:

cd build-dir
ninja web  # builds web site in build-dir/web/public
ninja preview-web # as above but also starts a server with a web site preview
  1. Tests covering your feature were added?
  2. Any new feature or behaviour change was documented?
Edited by Lars Bilke

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