Set up redirects

[maximlt]

We need to set up redirects as the structure of the new website changed compared to the previous one:

• Top-level links:
  • user_guide -> getting_started
  • make_project -> contributing
  • [removed] maintenance -> /
• Redirects, e.g. /attractors to /gallery/attractors
• Direct notebook links, e.g. /attractors/attractors_panel to /attractors/gallery/attractors_panel

Some useful links:

• https://sphinxext-rediraffe.readthedocs.io/en/latest/
• https://chrisholdgraf.com/blog/2022/sphinx-redirects-folder/
• https://documatt.gitlab.io/sphinx-reredirects/