Clean up docs

[tcompa]

(based on @ychiucco's work in the tasks repo)

[tcompa]

Based on work in the tasks/client repo's, we should also clean up the docs here.

• Prune any unnecessary dependency from both docs/docs-requirements.txt and {pyproject.toml, poetry.lock}
• Update GitHub actions:
  • Remove the docs part from the CI github action into an independent documentation.yaml, and also remove the creation of openapi.json
  • Copy the excat github action we have in tasks/client
  • Add a new preliminary job that: (1) install poetry, (2) set-up python, (3) poetry install --without dev --with doc, (4) create openapi.json. EDIT: we are now moving to pip, in the other repo's.
• Make the mkdocs.yml configuration (and docs folder) as similar as possible to the ones in tasks/client:
  • add extra.css, and the new css info in mkdocs.yml
  • make handlers/python options as similar as possible to the others
  • make theme/features as similar as possible (e.g. add content.code.copy)
  • make site_name specific ("Fractal Server"?)
  • add copyright footer, linking to LICENSE.md
  • anything else which is needed to have three homogeneous docs pages for the three projects

[tcompa]

The relevant openapi block will look like

      - name: extract openapi schema
        env:
          DEPLOYMENT_TYPE: development
          JWT_SECRET_KEY: 1234
          DATA_DIR_ROOT: /tmp/fractal-server/
          FRACTAL_TASKS_DIR: /tmp/fractal-server/tasks/
          FRACTAL_RUNNER_WORKING_BASE_DIR: /tmp/fractal-server/artifacts/
        run: fractalctl openapi --openapi-file docs/openapi.json