Add support for light mode

[troccoli]

This PR adds a theme switcher and supports for light mode. The default is still dark mode though. It has been done with the help of Codex AI.

• CSS variables have been added for both dark and light mode and used throughout.

• The logos in the Community section on the homepage have been left unaltered, as I would have had to research each company to understand how to correctly display their logo in light mode.

• By default the syntax highlight themes are dracula for dark mode and github-light for light mode. They can be customised by the TORCHLIGHT_DARK_THEME and TORCHLIGHT_LIGHT_THEME env variables.

For reference I have attached the plan I and Codex followed and the audit file used during the plan.

LIGHT_MODE_PLAN.md
LIGHT_MODE_AUDIT.md

Questions for the reviewer(s)

1. I have converted the homepage too, but I can revert that back if you prefer. I am interested mainly in the documentation part of the site.
2. I have used CSS variables as I said above, instead of the
   dark: class modifier. If you prefer using the Tailwind classes directly I can have a go at converting something like bg-[var(--theme-bg)] to bg-white dark:bg-zinc-900
3. I have taken the liberty to add my name to the footer. Let me know if that's ok