docs: audit improvements — SEO, accessibility, and theming polish

[mathuo]

Content & metadata:

• Add description frontmatter to all 52 docs pages that were missing it
• Fix navbar logo alt text ("My Site Logo" → "Dockview Logo")
• Fix sidebar typo ("Auxilliary" → "Auxiliary")
• Fix typo "mananger" → "manager" in advanced.mdx
• Add frontmatter to iframe.mdx which had none
• Replace Docusaurus template authors in blog/authors.yml with mathuo
• Delete old_docs_DELETE_SOON/ directory

Accessibility:

• Add alt text to 8 images in dnd/overview.mdx and floatingGroups.mdx
• Add :focus-visible states to buttons, github link, framework selector, icon buttons, and hero framework buttons
• Fix CSS display:' flex' syntax error (space before value) in 8 places
• Add prefers-reduced-motion support across buttons, hero animations, and spinner

Theming:

• Fix syntax token colors in light mode (contrast failures: #cf8cff → #7c3aed, #82aaff → #1d4ed8)
• Add explicit dark mode overrides for syntax tokens (previously unset)
• Fix spinner dot color (#fff → var(--ifm-color-primary)) — was invisible in light mode
• Define --ifm-hover-overlay variable for light and dark modes
• Fix invalid CSS: padding: none → remove property to restore Infima defaults
• Remove dead commented-out code blocks from custom.scss

[codesandbox-ci]

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Latest deployment of this branch, based on commit b1c0bdf:

Sandbox	Source
dockview-app	Configuration
editor-gridview	Configuration
externaldnd-dockview	Configuration
fullwidthtab-dockview	Configuration
iframe-dockview	Configuration
keyboard-dockview	Configuration
nativeapp-dockview	Configuration
rendering-dockview	Configuration