[docs] Default to following the system dark/light mode (#7888)
## Summary
This fixes the theme toggle so it has the most useful setting on by
default: following the system dark/light mode
See
https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/#automatic-light-dark-mode
## Test Plan
It follows the official docs exactly. If the docs get published for each
PR branch, we can also check there if it works as intended.
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Co-authored-by: konstin <konstin@mailbox.org>
