docs(site): add features, FAQ, troubleshooting, about, and dark-mode guide

[crypto-frog]

Expands the site from five thin pages to ten substantive ones, and links them together.

New pages

Page	Purpose
/google-sheets-dark-mode	Why the Sheets grid stays white under generic dark-mode extensions, what Force Dark Mode does and does not do, and what canvas interception changes
/features	Appearance controls, coordinated tints, header highlighting, profiles, backup, plus an explicit list of what the extension deliberately does not do
/troubleshooting	Symptom-by-symptom fixes, derived from TROUBLESHOOTING.md and KNOWN_ISSUES.md
/faq	Eleven questions with FAQPage structured data
/about	Licence, governance, distribution, privacy stance, known limitations

Structured data

The FAQPage schema now lives only on /faq. The overview page links to it rather than repeating it, so the same structured data is not published at two URLs.

Navigation

Primary nav: Overview, Features, Dark mode guide, Install, FAQ, Troubleshooting, GitHub. A secondary footer nav carries About, Community, Development, Privacy, Releases, Discussions.

Accuracy

Content is drawn from the existing repository documents rather than invented. Constraints preserved:

• no support, fix, compatibility, or release-date promises;
• no claim of Google affiliation, trademark note retained;
• Firefox explicitly not claimed as supported, matching KNOWN_ISSUES.md;
• the desaturated-colour limitation is stated rather than hidden.

Testing

• npm run check passes. docs/ is not in scripts/release-files.mjs, so the extension archive and checksum are unchanged.
• Documentation-only change.
• Rendered output needs checking after merge, since the Jekyll build only runs against main.