Add Common Errors reference page

[pepicrft]

Fixes #18

This PR adds a new "Common Errors" reference page at /en/errors to help developers quickly find documentation when they encounter Swift concurrency compiler errors.

What's included

The page covers 7 common Swift concurrency errors:

• "Sending value of non-Sendable type 'T' risks causing data races"
• "Expression is 'async' but is not marked with 'await'"
• "Call to main actor-isolated function cannot be made from a non-isolated context"
• "Actor-isolated property 'X' can not be referenced from a non-isolated context"
• "Cannot pass argument of non-sendable type 'T' across actors"
• "Task-isolated value of type 'T' passed as a strongly transferred parameter"
• "Cannot form key path to main actor-isolated property 'X'"

Each error includes:

• The error message pattern (with generic types)
• Clear explanation of what causes it
• Links to relevant documentation sections
• Quick fix hints

Implementation approach

This is Phase 1 of the feature request - a static, searchable reference page. Users can:

• Use Cmd+F / browser search to find errors
• Be indexed by search engines (SEO)
• Access from mobile without JavaScript requirements

If this proves useful (based on traffic/feedback), we can enhance with client-side fuzzy matching in a future PR.

Style

The page follows the existing site conventions:

• Matches the tone (direct, clear, no jargon)
• Uses consistent markdown structure
• Includes hero section like the main page
• Proper frontmatter and navigation

[pepicrft]