[web] Adding model hub page

Initial prototype, to be extended later.

Two new categories models and projects can be added to pages (benchmarks) as metadata and then those pages will show up on the corresponding project or model page.

Current projects shown: Decovalex and Eurad Current models shown: analytical, material, lab-, url-, field-scale models

Linked from docs overview page or at /hub.

Also: Proper web previews via real web server (instead of CI artifacts): better urls, images work. More info on GitLab blog.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?