Add dark mode support to docs

[atesgoral]

Summary

• add switchable light and dark Just the Docs stylesheets
• default docs theme to the user's system color scheme and persist manual preferences in localStorage
• add header icon controls for theme cycling and GitHub

Icon Sources

• GitHub icon: Simple Icons (github.svg)
• Theme icons: Lucide (monitor, sun, moon)

Verification

• BUNDLE_GEMFILE=/tmp/ruby-sdk-docs-bundle.gqtVn4/Gemfile bundle exec jekyll build --source docs --destination /tmp/ruby-sdk-docs-pr-build --config docs/_config.yml,/tmp/ruby-sdk-docs-local.yml --trace
• verified served header controls, SVG assets, and compressed theme toggle behavior locally

System

Light

Dark

Responsive

[koic]

Thanks!