docs migration - API Design Guidelines

Source: documentation/api-design-guidelines/index.md
Proposed disposition: Migrate — translate to DocC, break into multiple focused articles.
Proposed destination: api-guidelines/Sources/APIGuidelines.docc/
Reviewer: @swiftlang/language-steering-group

Note: The proposal calls for breaking this single large page into multiple articles within the catalog rather than a one-to-one translation. Plan the article breakdown before beginning the translation work.

Checklist

• Create a GitHub issue to track this migration
• Determine disposition (suggested: Migrate) and choose explicit destination
• Plan the article structure — identify logical sections that become standalone DocC articles (e.g., Naming, Conventions, Special Instructions, etc.)
• Review source for accuracy against current Swift language and standard library idioms
• Translate Jekyll markdown to DocC markdown, one article per logical section
  • Remove YAML front matter from the source file
  • Convert any Jekyll-specific markup (liquid tags, includes) to DocC equivalents
  • Verify all code examples compile under Swift 6
  • Update cross-references between the new articles using DocC symbol links
• Register all new articles in APIGuidelines.docc/Documentation.md
• Run swift package generate-documentation --analyze --warnings-as-errors in api-guidelines/
• Preview locally with swift package --disable-sandbox preview-documentation in api-guidelines/
• Submit PR to this repository for language steering group review
• After merge and publication: add redirect_to: in the swift.org source file's front matter
• Submit redirect PR to swift-org-website