docs: add complete GitW3 user guide

[coodos]

Summary

• add a nine-page GitW3 guide to the Docusaurus site
• document W3DS-only sign-in, AaaS profile mapping, and Git authentication
• cover new-platform creation and the staged existing-application port flow
• document the platform manifest, W3DS workspace, releases, PPA, and deployment identities
• add sanitized screenshots captured from the production GitW3 UI

Validation

• pnpm --filter docs build
• build succeeds; it reports only six pre-existing broken anchors in the W3DS glossary/W3ID pages
• verified no browser/session credentials are present in the added files

[coderabbitai]

Important

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Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 9710da28-f2a9-4fe8-aad8-0019d1c606d7

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