Refactor McpInstallButton to use Docusaurus/Infima styling

- Replace inline styles with Infima CSS classes (dropdown, button--primary, badge--success)
- Use Docusaurus theme icons (@theme/Icon/Copy, @theme/Icon/Success) instead of custom SVGs
- Add MCP logo icon and "Install docs MCP" default label
- Sort MCP clients alphabetically by display name
- Use CSS variables for easy theming customization
- Add dark code blocks for consistent appearance across themes
- Mark @theme/* imports as external in tsup config for runtime resolution
- Add type declarations for Docusaurus theme icon modules

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: scalvert