Docs: Add dark mode and a toggle

[Copilot]

Bnd documentation does not have a dark mode and it's a bit hard to read at night. So I let copilot add it.
It this might be obsolete once we switch to a newer theme - what we plan to do - but who knows when this will be. After all it wasn't that much, and it makes it a bit easier to read in the dark after all.

Screenshots

Light	Dark

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• github-production-user-asset-6210df.s3.amazonaws.com
  • Triggering command: /usr/bin/curl curl -L REDACTED -o /home/REDACTED/work/bnd/bnd/docs/img/front-page-dark.png --progress-bar (dns block)
  • Triggering command: /usr/bin/curl curl -sL REDACTED --max-time 30 --max-redirs 10 -o /home/REDACTED/work/bnd/bnd/docs/img/front-page-dark.png (dns block)
  • Triggering command: /usr/bin/python3 python3 -c import urllib.request url = 'REDACTED' try: with urllib.request.urlopen(url, timeout=30) as resp: data = resp.read() with open('/home/REDACTED/work/bnd/bnd/docs/img/ (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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💬 Send tasks to Copilot coding agent from Slack and Teams to turn conversations into code. Copilot posts an update in your thread when it's finished.

[chrisrueger]

Will be replaced by #7150