docs: Integrate Fer, FON, Next Web, and reference modules

[Fuyeors]

Summary

• Integrate the existing Chinese Fer and FON specifications from Fuyeor/fer-spec into content/fer and content/fon as independent modules.
• Add the real fer-spec/web/next-web/zh-hans.md draft as content/next-web/overview/zh-hans.md; no new generic Web API content is introduced.
• Add the reference module with editing guidance for structure.json, Markdown, and the PR workflow.
• Add localized module indexes and source structure.json files for all four modules.

Documentation scope

• English article files are intentionally not added yet.
• Fer/FON source headers containing <!----> or 状态：草案... are removed during integration.
• Existing cross-links are updated to the new content/ module paths.
• structure.json navigation matches the actual directory layout.

Validation

• Full content generation completed successfully for all modules.
• pnpm test passed: 6 test files and 26 tests.
• git diff --cached --check passed before commit.
• Audits confirmed that no generic Fetch/Storage/DOM Web API pages or old fer-spec/web/next-web links remain in the migrated modules.

Notes

The repository does not currently define a lint script for the selected workspace packages, so pnpm lint is unavailable and was not used as a passing check.