Add gil-inspired virtual module API pattern recommendations

[srdjan]

Motivation

• Provide high-level ergonomics for zigttp:* virtual modules inspired by ivanleomk/gil so handlers stay concise and declarative.
• Encourage module APIs that hide low-level Zig/runtime details and surface one-liner happy paths with optional options objects.
• Standardize patterns across modules (fetch, auth, cache, validate/decode) to improve discoverability and consistency for handler authors.

Description

• Add docs/design/virtual-modules-gil-inspired-patterns.md containing recommended API patterns, examples, error-surface guidance, anti-patterns, and a review checklist.
• Update docs/README.md to link the new design note under the Design And Roadmap section.
• The design note documents concrete recommendations such as one-liner helpers, typed convenience methods with escape hatches, response helper objects, literal-friendly configs for analysis, build-time registration vs runtime use, and naming conventions.

Testing

• This is a docs-only change and does not modify runtime code or tests.
• Verified repository state with git status --short and created a commit for the two modified files successfully.
• Inspected the new file and updated README with nl/sed to confirm contents and link insertion, and those checks succeeded.

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Codex Task