Doc upgrades

[thorstenhater]

• Polish & modernise two cells + gap junction example.
• Investigate problems with mathjax
• Update spike data format docs.
• Switches theme to FURO, which is better maintained and document (like ... at all)
  • with the new theme, we gain dark mode, yet we need to fix our graphics to make it a really nice experience.

Fixes #2227
Fixes #2228

[AdhocMan]

Looks great!
As you mentioned, some images still need work.
These are the ones, where I've found issues with the dark theme:

• https://arbor--2229.org.readthedocs.build/en/2229/tutorial/network_two_cells_gap_junctions.html#
• https://arbor--2229.org.readthedocs.build/en/2229/ecosystem/index.html#simulators
• https://arbor--2229.org.readthedocs.build/en/2229/concepts/recipe.html#an-example-model
• https://arbor--2229.org.readthedocs.build/en/2229/concepts/hardware.html#hardware-context
• https://arbor--2229.org.readthedocs.build/en/2229/concepts/domdec.html#domain-decomposition

For the html based figures, it should be enough to change the text color to always be black. Maybe through some styling option specific to each div class?
The SVG figures could be fixed by setting the background to always be white. There appear to be ways to enforce this directly inside the SVG file.

[thorstenhater]

@AdhocMan thanks for putting in the leg work and finding those issues. Should be all good now.

[AdhocMan]

Looks great now!